I've been using attachment_fu to upload images in a rails application. Everything was working well except the thumbnails weren't generating. I think everyone follow's Mike Clark's excellent blog:
He doesn't mention that the parent_id is required, so I thought it was simply the id of the model to which the photo/upload was attached.
After some digging, I found a conversation here:
Turns out attachment_fu, creates a new entry for each thumbnail, then strings them together with the parent_id. So, make sure you keep the parent_id column otherwise attachment_fu silently fails to generate the thumbnails.