Schema
ImageObject schema
Also: Schema.org ImageObject, image schema, before after image schema
Gallery images are often the most visited part of a visual business's site and the least described. A caption that states the procedure or project, the materials or technique, the timeframe and who did the work tells people and machines what the image shows.
ImageObject needs no JavaScript. It sits in the JSON LD @graph with the rest of the page, and every caption must be accurate and consented to by the client shown.