Thanks for reviewing this patch. I agree that we should find some way to keep aspect ratio of images. Let me try it later.

For those larger wallpaper assets, the solution now is: image files named with ~@2x will replace original ones at build time, if we build with $ HIDPI=1 make. Thus, so far, there are no works needed for individual apps on replacing images. These makefile changes are addressed in the earlier system PR. I think we need some documents on it though. For now there are just some descriptions on gaia/README.md.