Preferring PNGs over JPEGs ...

Hoping to get crisper images for the mobile phone by switching to PNGs.
vault.mk and compile.sh are now slimmer / simpler.

mobile.black.png will be re-generated. Surprisingly, file size was
larger with transparent background. Hence, not making png transparent.

None of the old files will be touched. Only new ones generated.
mobile.black.png will be re-generated.
Merge pull request #784 from abhinavc/master

Preferring PNGs over JPEGs