Use JPEG for uploaded images to make much smaller files.

The photos in JPEG format are as little as 1/10th the size of their
PNG counterparts, allowing for much more reasonable page load times
for sightings.

Changing to JPEG required some fixes to PIL, which were available
via Pillow, a fork of PIL providing a drop-in replacement with
more frequent fixes.

To upgrade, delete PIL first: pip uninstall PIL, then either run
dev/setup (full install of all packages) or pip install Pillow
(install Pillow only), then verify in shell: import PIL. (#566)