Best instagram niche to start?

Basically any page that you can upload half naked girls.
I run tattoos page and I get almost twice more engagement on posts with tattoed girls than i get on men tattoos.

is there any risk using images that are not royalty free?

The best way is to pick a large, evergreen niche and the then dig deeper, be specific. Random example for porn niche (don't do that on insta tho): porn ----> teen ----> skinny---> Group sex (???) ----> anal (?????). That's your micro niche: "skinny teen Group anal sex". I hope that makes sense lol, you can do that for every niche; Sorry for this drastic example

I would say that the beauty niche is one of the best... lots of money to be made in it with affiliate marketing/dropshipping and you can grow followers quickly because ladies LOVE this stuff. Got convinced to do it from a YouTube video and I’ve recently opened some makeup accounts! Here’s the vid I saw...