File names are utterly important and a known secret of SEO houses.
If you name a image of a puppy - image124.jpg Google doesn't know what that image really is or what it's relation is to your website.
Now same picture of the puppy if you name that picture - puppy-playing-on-grass.jpg and the alt text is relaxing puppy and the content on the page is about a puppy trust me Google can safely say that is most certainly a picture of a puppy.
When it comes to SEO, it's important to use acceptable keywords to help your webpage rank on search engines. Creating descriptive, keyword-rich file names is absolutely crucial for image optimization. Search engines not only crawl the text on your webpage, but they also search for keywords within your image file names.
A good habit to get into is to look at your website analytics, and see what phrasing patterns your customers use to perform searches. Determine the most common naming patterns they use, and apply that formula to your image naming process.
If you are not going to get that detailed, just be sure to use good keywords when naming your images.