Added rudimentary 'do the last command in that pane' script and bind for tmux. Not very slick but does what I want in the basic case.
Seperate out the root of the tmp to make it easier to switch to a ramfs.
Use an existing ramdisk for imgdiff.
Ctrl+H to show the 'history' in Vim? Worth a try, although it does of course mean something else to most people.
Imgdiff: cope with adding/deleting images (i.e. one of the comparison images is /dev/null).
Also remove unnecessary subimage search as images are guaranteed to be the same size.
Make sxiv the same width as the image itself.
Make sxiv scale down larger images by default.
Clean up temp dir when finished.