Gimp is pretty bad. Paint.net is much better, but I use photoshop.
also I have a problem. I've been trying to set up the rig for my games main character, and I don't see how you can mirror the weight paint onto the other side on the other bone.
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You can have Blender apply the weight automatically.
-While in weight paint mode, select a bone and press 'w'
-Select one of the options from the drop-down menu that appears. (I usually pick the second option, but they both work. I suggest trying them both, to see which one works best for your mesh.)
You will need to do this for each bone.You may also need to touch up some bits.