Image captions (SEO)

I'm not completely sure what an image caption is when it isn't a title. Maybe title is just an HTML keyword, and caption just means the text that is displayed above or below an image, regardless of what technical implementation is used for this.

When using Yoast, make sure to include the Focus Keyword in the caption.

• Met caption wordt hier de tekst onder of boven een afbeelding bedoeld
• Bij voorkeur inclusief de focus keywords

yoast.com/image-seo:

The image caption is the text that accompanies the image on the page — if you 
look at the images in this article, it’s the text in the gray box below each 
one. Why are captions important for image SEO? Because people use them when 
scanning an article. People tend to scan headings, images and captions as 
they scan a web page. Back in 1997, Nielsen wrote:

   “Elements that enhance scanning include headings, large type, bold text, 
   highlighted text, bulleted lists, graphics, captions, topic sentences, 
   and tables of contents.”

In 2012, KissMetrics went even further, stating that:

   “Captions under images are read on average 300% more than the body copy 
   itself, so not using them, or not using them correctly, means missing out 
   on an opportunity to engage a huge number of potential readers.”

Do you need to add captions to every image? No, because sometimes images serve
other purposes. Decide whether you want to use yours for SEO as well or not. 
Bearing in mind the need to avoid over-optimization, I’d say you should only 
add captions where it would make sense to the visitor for one to be there. 
Think about the visitor first, and don’t add a caption just for image SEO.