[Update] Approve "Performer Image Size"

[AdultSun]

Needs official approval: https://guidelines.stashdb.org/docs/performers/performer-images/#performer-image-size

Haven't seen any pushback to the language in this one. Main thing here is deciding if we should enforce a minimum image size when other photos are available and what that size would be. Images are already removed for being too small, we just don't have a hard line saying when that is or isn't appropriate.