3 ways of optimising images for google images
If you want your images to appear in google image search, you must optimise them. In order here’s what I believe to be the 3 best ways.

This image of St Mark’s Basillica has been optimised
1 - Optimise the filename.
After reading search engine journals article on image search i realised why my images don’t rank in google images, despite my getting the title and alt text correct. My filenames were totally random. Looking at some of the top ranking images for a search term such as “animal collective” all the top 5 results have a filename of “animal_collective.jpg”.
2 - Optimise the surrounding text (or caption)
After the filename, the surrounding text seems to be key. That will likely be the caption, so if you don’t do captions, there’s a good reason for starting.
3 - Optimise the alt tag and title
Get the keywords in the alt tag and title (if the image links somewhere). Then you can be sure all bases are covered. Take care to be sure the alt tag is still accessible though.