Official Google Webmaster Central Blog: 1000 Words About Images

[HTMLBasicTutor]

Information about images and Google:

But how does Google know that the images are about coffee and not about tea? When our algorithms index images, they look at the textual content on the page the image was found on to learn more about the image. We also look at the page's title and its body; we might also learn more from the image’s filename, anchor text that points to it, and its "alt text;" we may use computer vision to learn more about the image and may also use the caption provided in the Image Sitemap if that text also exists on the page.

To help us index your images, make sure that:

•we can crawl both the HTML page the image is embedded in, and the image itself;
•the image is in one of our supported formats: BMP, GIF, JPEG, PNG, WebP or SVG.

Additionally, we recommend:

•that the image filename is related to the image’s content;
•that the alt attribute of the image describes the image in a human-friendly way;
•and finally, it also helps if the HTML page’s textual contents as well as the text near the image are related to the image.

Read the whole post here: Official Google Webmaster Central Blog: 1000 Words About Images

[Band333]

Great sharing. It raise few point to optimize images search:
1. Page content
2. Anchor text
3. Alt text
4. filename
5. Text within images (algorithms index images, e.g. a OCR tool can recognize images. I suspect this is contribution from mobile user, Google Goggles apps)