I'm part of the Simple Pin Pinterest Strategy Group on Facebook where Kate Ahl of Simple Pin Media dispenses some valuable Pinterest insights.

Kate Ahl often posts these two articles in response to people asking about keywords and SEO:

(1) http://www.simplepinmedia.com/how-to-keyword-on-pinterest/

(2) http://www.simplepinmedia.com/seo-and-pinterest/

And I read some discussion in the comments about whether to use the “alt” attribute or Pinterest's proprietary “pin-data-description” attribute for your default Pin descriptions. And in one of the comments, Kate links to this resource: https://kimberlyherrington.com/seo/seo-and-pinterest-descriptions/

And in that resource, Kim Herrington gives this example of optimal code for an image:

I'd like to expand on this from an SEO perspective because I've recently been figuring out how to best utilize these tags for my blog.

First I'll say that for me, SEO is top dog and will always take precedent over anything else including Pinterest. I will never do anything to benefit Pinterest or another social platform that will hurt my SEO. Luckily though, I'm discovering that I don't have to.

What is image ALT tag?

The words you place within the alt tag is what appears in lieu of an image when it cannot be displayed. It's also what is read aloud for blind people who are browsing the web. This is not a place for keyword stuffing or overly long Pinterest descriptions.

Google says the following about ALT tags:

https://support.google.com/webmasters/answer/114016?hl=en

Filling alt attributes with keywords ("keyword stuffing") results in a negative user experience, and may cause your site to be perceived as spam. Instead, focus on creating useful, information-rich content that uses keywords appropriately and in context.

So as you can see, a problem arises if you use your ALT tags as your Pinterest descriptions. And that problem is that you SHOULD "keyword stuff" a little bit with Pinterest, or at least append a few to the end of your description, but you SHOULD NOT do that for Google. Well, put even better, you should not do that for your USERS. Don't be a jerk to blind people and make their audio browser read a 300 character Pinterest description instead of a short actual alternative description of the image.

There is a limit to how long your ALT text should be but very few reliable sources of information on exactly what that limit is. Google does not comment on a limit directly. But one number floating around as a limit is "16 words". You can read more about how Shaun Anderson ascertained that limit here:

https://www.hobo-web.co.uk/how-many-words-in-alt-text-for-google-yahoo-bing/

In my experience for SEO purposes, I don't go near 16 words. Examples of my alt tags are "Pollution is heavy in Hanoi Vietnam" ... "Sunsets are breathtaking in Boracay Philippines" ... "GoPro Hero 5 Review & Comparison".

So you can see that this rule of thumb for ALT tags goes against best practices for Pinterest descriptions and therefore shouldn't be used as such. More on that in a moment.

What is image TITLE tag?

The title attribute is shown as a tooltip when you hover over the image (except for Internet Explorer which still shows ALT tag as the hover over text, and Microsoft Edge doesn't seem to show anything). So for example, if your image leads to an Amazon product, the pop-up title tag could read, "Buy the GoPro Hero 5 on Amazon". This is also not a place for keyword stuffing, but, it is a place to insert a different keywords than what was in your ALT. At minimum, it should just be the same as your ALT attribute if you don't want to get too fancy.

What is the image DESCRIPTION tag?

If you add text to the description field that text will be stored as post content for the attachment post. That means if someone lands on the attachment post page for the image they will see the long description (provided the current theme supports it). By default, WordPress often links the image to the attachment page. I always change that to a relevant link (like to the Amazon product the image references), or I remove the link all together.

Though the Description field does not have the Visual (WYSIWYG) editor you can treat it exactly as you would any other post or page. It takes HTML and allows you to format the content to make it look like a regular post. Obviously you can see that this is another great opportunity to optimize your images for search that isn't limited in length like the ALT or TITLE attributes.

You can learn more about this here:

https://mor10.com/power-of-wordpress-image-metadata/

Keep in mind though that this DESCRIPTION image attribute has nothing to do with Pinterest. So let's jump into Pinterest now and how all this relates.

What is the DATA-PIN-DESCRIPTION tag?

As Kim was showing you in her example of optimal code above, the DATA-PIN-DESCRIPTION is essentially code that Google doesn't read, but Pinterest does read. If you create a special DATA-PIN-DESCRIPTION for your image, that is what will appear as your Pinterest description automatically when someone Pins that image from your site.

As of now, I don't create a separate Pinterest description for EVERY one of my images on every post. I only do it for the special Pinterest friendly graphic that I make for each post, because chances are, that's the one that people are going to Pin from my site (at least I hope).

Pinterest says that, "Descriptions can be up to 500 characters. While only 75-100 characters of your description will appear in grid view, the entire description will appear when Pinners click on a Pin."

You can learn more about Pinterest attributes here:

https://developers.pinterest.com/docs/widgets/save/?

Like Kim was showing you, there's also the DATA-PIN-ID attribute which treats any new Pins of this image as repins of the original. Doing this can give you a better feel for engagement, because any Pins you create will count towards repins of your original Pin.

Then there's the DATA-PIN-URL which overrides the image's page URL and substitutes the URL of your choice. You should make sure this is the URL of the specific blog post and not the URL of your blog's homepage so that people get to the correct place when they click on your Pin.

There's also the DATA-PIN-MEDIA which overrides the image and substitutes a different image in the Pin Create form. You can use this to have (1) a compressed version of the image on your blog post which reduces page load time for your visitors, and (2) a separate higher resolution version of the image that's hosted on your website, doesn't appear on the post itself, but is pulled up instead when someone Pins the compressed image. You could technically use this feature so that when someone clicks Save on one of the other 20 images on your post, the DATA-PIN-MEDIA queries your special vertical Pinterest friendly graphic. Probably not the best move because you want to allow people to Pin the image they choose that's relevant to whatever board they're saving to, but technically it's possible to use this attribute for that purpose.

And there's quite a few more Pinterest specific attributes but those are the main ones I'd concern myself with.

So should I use the ALT attribute or the DATA-PIN-DESCRIPTION attribute?

Well, I'd hope you see by now that you should use ALT the way it's intended, to accurately describe the image in as few words as possible, and DATA-PIN-DESCRIPTION as it's intended, which is to write an eye catching Pinterest-specific description. If you were to write up to 500 character descriptions as your ALT tags (or even 75-100 character descriptions), Google may consider your website spammy and penalize you. However, you should write the best Pinterest descriptions possible. So for these reasons, DON'T use ALT attribute, and DO use DATA-PIN-DESCRIPTION attribute for your default Pin descriptions.