Search engines read the image's file name as a ranking clue. "IMG_4821.jpg" tells Google nothing; "blue-running-shoes.jpg" tells it exactly what the image is.
The rules
- Be descriptive — use real words that match the image.
- Use hyphens between words, not underscores or spaces (
red-leather-bag.jpg). - Lowercase only, to avoid case issues on some servers.
- Keep it short and avoid keyword stuffing.
The full picture
File naming works together with alt text, the right format, and a compressed file. To get found in image search, also see ranking in Google Images.