I recently found that notifications in the name of an application just showed up with a question mark graphic instead of the application icon. Researching the forums it seems that that is a widespread problem. Yet a solution was quite simple:

The facebook app view from the developer's dashboard

On the application’s dashboard just select ‘View application profile’. Once there, upload a picture (can be the same picture, remember 75x75px) via the ‘Change picture’ dialog.

The application profile

Done. Yoho!