I couldn't miss the chance to practice my drawing!
Using the Imp spell on a monster that has Invisible status or having an enemy absorb Imp with Enemy Runic will always cause a monster's graphic to change to that of an Imp or vice versa, even if they are immune to the status ailment. In the same vein, a monster's graphic cannot be changed to that of an Imp simply by causing the status. Imp can also do this to a character using Runic, but in that case, the Imp sprite only lasts for the duration of the spell. This all has to do with the fact that only the Imp spell itself can change a character or monster's graphic to an Imp directly and the fact that Imp status doesn't automatically cause the change in monsters.
A variant to the previous mentioned bug, the status Imp, while it will change a character's sprite automatically will not change a monster's automatically. A prime example of this is Bad Breath; while it can cause the status, the player won't visually know it's been inflicted until the monster takes a turn or two depending on the monster. Another example of this can be found with Tonberry and Tonberries. The Tonberries tends to walk forward every few turns, then return to their original position using a fade in/fade out animation combination. If the player inflict one of those with the Imp status, and it uses an animation to move back to where it started, the Imp sprite is lost even though it is still inflicted with the Imp status. This all has to do with the fact that only the Imp spell itself can change a character or monster's graphic to an Imp directly and the fact that Imp status doesn't automatically cause the change in monsters.