Do you think that image alt. text is being read as google as part of the in-body text? I am not sure how its weight compares. Is it calculated as part of the needed density for a keyword?
For art galleries, image search indexing is more difficult. The images have few attributes. The artists rarely have a textual description of the work beyond the title, size and materials used. This leads to pages having one image with very little text. The gallery I work with sells very small issue quantity limited edition canvas reproductions by contemporary artists. If the artist is represented by several galleries, how do you differentiate an image to have it indexed? What makes the same image, provided by the artist or the publisher, "better" on one site than another?
100% correct, I have done a significant amount of research on image optimization; 1) reduce the size, using yahoo smush, or punypng.com (not for seo but for site performance, which is becoming more and more important...) 2) the actual image name does not mean squat, I assumed it would - this has been tested and I have backward engineer the results. 3) Alt text still has some value, but its high depreciated, everybody got on the badwagon. 4) the text in close proximity (testing shows 67 characters) matters! 5) captions are great and have actually proven to show in image SERP's, even if the image did not have relevant alt text. Anycase, just my 2cents.