DPI is senseless for digital artworks. DPI uses only for print. For digital artworks you can choose free any resolution what do you need for work. PNG/JPG. PNG is a format with lossless compression. If you want to avoid compression artifacts than you should use PNG. If you just want to present your artworks then 100KB JPG images will be fine.

Like others said 72 ppi is the standard (and yes it’s important otherwise your website will take forever to load). I’d recommend you post pictures of your artworks kinda staged (like printed and then staged at a small angle) so it’s harder to just screenshot and steal 😐 if you don’t wanna print then id follow some of the other advice about glaze or something! ✨