git config --global mergetool.gimp.cmd 'gimp --new-instance "$MERGED" "$LOCAL" "$BASE" "$REMOTE"'
git config --global alias.gimp 'mergetool --tool=gimp'
Then you can
if you have conflicts in image files. It will open four of your image in Gimp:
+ The actual file you should copy changes into.
+ The LOCAL branch's version of the file.
+ The BASE version... from before the branches diverged.
+ The REMOTE branch's version.
Save your merged changes to the actual file and close all 4 Gimp windows and tell Git that (y)es the merge went ok. Unless it didn't!