Had a quick look at the .deb file and at first glance everything looks ok. There is a postrm shell script which includes the lines to delete the images folder. But I wonder why this file is called at upgrade. As far as I know an upgrade is something different from an uninstall-install-path. Maybe some other developer can help out at this point?
But nevertheless it would be better moving the images folder to MyDocs or at least to include it in osso backup and to leave that folder untouched when uninstalling. A message to the user indicating that there are files left could be displayed but usually it's not the developers job to clean up users directory. Debian packages usually leave config files and user generated files untouched when uninstalling so user can decide by himself what to keep and what to delete.

Please change that behaviour!