Do not post "funny" dialogues. Do not post comments with lines the characters in the picture are supposedly saying. There's nothing wrong with funny or witty comments, but those are neither.
What else is wrong with dialogue comments? They may not quite be funny or witty, but why can't we post such at ALL?
I wasn't around when that rule was made, but my personal stance on it would be that I don't really care what roleplay everyone has in their heads for characters in images. Not to mention it will invariably lead to fights about whether one character would act a certain way or say a certain thing because people are incredibly defensive of their headcanons. Not to mention that, without Expand tags, they take up a massive amount of space doing not much relevant to the art gallery and not really communicating anything.