The number of images on my hard drive is spinning out of control and I never got around to using dedicated software with image tags for my images (maybe I should). Lately, I am paying more attention when naming an image file. And the one thing that helped me most is to put the noun first. For example:
  • sky_sunny.jpg
  • sunny_sky.jpg. 
The latter is how we are used to think, the first is best for sorting and finding things on a computer.