I would say gimp 2 is a good program, mainly because you can put shading presets over them, for example if you were a beginner learning to shade. I have used SkinEdit (Can only do universal format skins) and McSkin3d, and I enjoyed them both, but gimp 2 is my personal preferance

I don't use it for posting skins because it's against the rules, but I personally use NovaSkin for changes to my own skin I need right the second. You don't need to download anything, you don't download files, no. You make your changes, then press apply. It's as easy as that.

Photoshop is technically the best program for design, but most people use that MCSkin3D thing. I tried it, and Photoshop was just better. Use what you want, but I'd recommend Photoshop if you have it. Don't buy it just for skinning, though.

If you aren't very experienced with skinning MCSkin3D can be a little tricky, but when you realize how to use it it's just amazing. It doesn't have gradient tools etc. but it is SO much easier to pick out colours and place pixels on, since it has a 2D view, 3D view and an option to see both at once.

If you're brand new to skinning I can link you a few colour/shading tutorials if you'd like?

MSPaint... just kidding

most people use MCSkin3D by Paril