keyboard/altgr: automatically generate thumbnails from source images

Treat original images as untracked data, named with proper conversion
parameters for results similar to earlier gimp manipulation and exports.
Reuse the mkimgthumb script to crop and resize as appropriate:

	tools/mkimgthumb --jpg '400x300>^ x200<' *

This will result in similar visual quality (at much smaller file sizes)
as gallery cells do not exceed 400px wide.  Enlarge to at least 200px high
to elicit preview images on social media.