From working your website design, to using the image elsewhere such as in print, you need to make sure you have an image that can be used in a variety of locations, which isn’t always possible, but best try to plan for it.
If you are creating thumbnails for use on a website, then include a common word such as thumb as part of the file name. That way when you are search through an image library for those thumbnails, you can search via a keyword!
Also by renaming images files using keywords related to the actual image, you are improving the SEO of the image. When inserting images into a blog post or page, don’t forget to fill in the alternate text and caption if available.