When inserting multiple images at the same time, we could deadlock -- inserting

new rows into images would (due to the foreign key constraint) lock the corresponding
row in event using a shared lock, then the update (due to last_picture) would
upgrade the lock to exclusive, causing a deadlock later. To fix this, we move
last_picture into its own table; we could also lock it explicitly earlier, but
that would stall other transactions (and they can run for 10-15 seconds or so,
or perhaps even longer) so it is not desirable.