Convert and resize big photos

Images should probably be checked for their resolution and resized down if needed (to ensure that mom and pop don't save 12 megapixels images in there while we display them at 256x256 or something :)

The stakes:
* Performance (dynamically resizing a bunch of huge images everytime we update the list would probably be cpu-intensive)
* Disk space (what if the user only imports 12 megapixels RAW images? :)

The big question:
* What should we resize down to? while we currently only display tiny icons, maybe the user would like to see a slightly bigger/detailed version on mouseover.