you should definitely ask. but of course, most people don't care about all the time and effort fanartists put into their work, so you usually won't see credit on fanart icons, let alone people asking for permission.

also i think it's weird to use fanart as a PB, and not acceptable to use someone else's OC as a PB for your OC.

(icon/fanart/cc wank in 3... 2... 1...)