That is a quicky, so long as you understand how to use the tools in any photo edit program it's simple, the only reason it should take time is if you are stuck for ideas and or changing things as you go. I can't say I am a pro or anything of that sort, especially considering I haven't posted that much, but I can say I have a good understand of photo programs, as well as a lot of software in general. Patience and understanding is key to anything.
Edit: Of course having lots of free time would help when it comes to making lots of skins(good or not)