In general, it's blunt and dismissive. When you post a gif/pic without any explanation, it often comes across as, "you're argument is obviously wrong, I shouldn't even have to explain why. You should be able to tell from this pic just how wrong you are." On the other hand, taking the time to write an actual explanation (saying what exactly is going on in this pic you posted and why it refutes the other person's argument) is a sign you understand that everyone sees things differently, an that what seems immediately obvious to you doesn't necessarily seem that way to other people. It's a sign that you're willing to engage the other person in a dialogue, that you're willing to listen to what they have to say. Posting a pic without any text often comes across as simply shutting the other person down.
In short: it's a way of showing respect for the other person.