What's the practical way to name images for seo?

One thing I'd avoid is renaming images that are already live and getting image search traffic. New filename means a new URL, so you more or less start over for those. Good habit for new uploads, but leave the old ones alone unless they're genuinely awful.

Rename before you upload. Lowercase, words separated by hyphens, describing what's actually in the picture, so something like red-leather-office-chair.jpg instead of IMG_4821.jpg. Keep it short and don't cram keywords in. The alt text matters more than the filename anyway, so spend your effort there.