www.teambg.com have tools to edit all but one of the BG2 image files (The one they cant is the plt file used for portraits). However, I dont think its possible to add in avatar pics, I think they have to be swapped with existing ones. Its a limitation of 3rd party understanding of the infinity engine rather than the engine itself though, so this may change.
If you decide to try, youll probably need a 3d modeller to create the character and then render from that - it would be nigh-on possible to draw the character in 2d, given the number of frames and angles youd have to do.