Thinking logically, why would it matter all that much in the first place? Should Google promote websites simply because they've taken the time to add keywords to an image?
The answer is a resounding "no". Optimising your images is a good idea, but not for SEO. It's good for accessibility, which is far more important than SEO. Optimising images for SEO outside of your alt tags and good naming conventions is a complete waste of time.