The best thing to do is to rename the
.shotwell folder in your home folder, as that contains all the thumbnails and library information. Simply rename the current
.shotwell.backup and then load up
shotwell. It will regenerate a new
.shotwell folder and you will be able to start again, and you will also still have the old dot folder if there is anything in there that you want. (Make sure you click the option to enable hidden files in your file browser).
The reason why uninstalling a program and then installing it again sometimes doesn't clear up any problems properly is because the dot files of that program in your home folder are not removed. Therefore the new installation will still be using those files; purging a program only removes configuration files in the system folders.
Shotwell 0.13 (the default version in Ubuntu 12.10), the photo database file is in
~/.local/share/shotwell/data and the actual photo thumbnails are in