You can use whatever program to paint, but use ones, that can give you quality tools
like Photoshop, that's in my opinion. After you finish painting go to "Tools" link on hive workshop, it's right of "Spells". Now find tool BLP Lab, or Button Manager, open your picture with that program, select border, and you're done, you'll get your picture in 64x64 in blp format with borders, but it's cool to resize your picture on your own, f.e. in PS, You make a picture 256x256 you should resize it into 64x64 because all icons need to be like that