An Image Derp is an error where when 2 users posted post images at almost the same time. Because of how the site names images internally, sometimes one image will be "overridden" and replaced by the other. Usually, this manifests itself as an image appearing "actual size" instead of thumbnailed, or a thumbnail that is different than the actual picture
Around late May 2011, the site's code was changed to make image derps very infrequent. Some users had become fond of the derps, as their randomness and ability to instantly derail threads with no warning were a perfect match for the /oat/ board.
In late June 2011, image derps began appearing more frequently once again, leading some to wonder if they were going to return for good, or were merely an accident that would be fixed.
On June 28th 2011, site admin !!Celestia confirmed that the image derps would in fact be staying, though only on the /oat/ board. These derps are intentional and change the filename to 'derpy was here.' There are some reports of unintentional derping on other boards, but no claims have been verified.
At one point, a very small image of sad Pinkamina managed to override, or derp, all the way through the system to replace the base spoiler image. Due to some spoiler image gags and general confusion, this created a minor uproar but was eventually fixed by !!Celestia.