Write images out to temporary local files before writing them into storage

Testing more with S3, I noticed that sometimes we'd end up with garbage files
that were 4 bytes long instead of the thumbnail. I ended up tracking this down
to the fact that the file abstraction provided isn't very good about multiple
write+tell cycles, which PIL will sometimes use.

This change alters the image tags to write out to a temporary file, and then
dump the whole file into storage. This works much more reliably.