Here is MY definition and it will stand until a higher court rules otherwise. (Abbie being a higher court.)
An image thread is one made to show images and try and get them seen. For instance, "Sunset Images." Or "XYZ Art." Or, Pick your favorite image from the artist above. Some images used to illustrate a point within a broader topic will NOT be considered an image dump thread by myself, NOR will a thread welcoming a new member where the OP might "OH and AH” about the new member's art.
Furthermore, no one is going to take a break or get banned for an accidental posting of such a thread. It will simply be closed. Now, if someone decides to back seat moderate because THEY think it is an image thread, they may or may not take a break depending on how I see it.
There is your ruling for now.