Definitely not easy - you would need to be from a solid medical foreign medical school first, well-rounded person, lots of ancillary activity (not only >250/260++ board scores) like published research (in well known sources) , derm-related work/service projects, maybe if you attended some derm conferences, get your face indirectly known through networking - that is something. Some derm student societies might be open to IMGs also...
A few exceptional imgs/imgs have gotten it yearly, but we are talking less than a handful at most. In the past, people tried going from path>dermatopath>derm route - lots of years though. Not sure on re-enrolling/applying to US med school if you already graduated/passed usmle's though - know of a few that started from scratch (without taking boards) just to get a speciality like derm... very rare.
A fair question to ask - even if img or not...