So I used to use Kaleidoscope to visually inspect changes in PNGs before committing them. To do this from sourcetree I set up kaleidoscope as my visual diff tool and I would right click the images in sourcetree and choose "External Diff"
Recently I've noticed that this behaviour has regressed and no longer works!
No idea if the issue is with kaleidoscope or git or sourcetree, but I know that kaleidoscope can show image diffs through its CLI so I think it's more likely sourcetree...
Hopefully this shouldn't be too hard to fix if it's worked in the past?