When you page someone using an image, with HTML img tags, there is a delay between the messaging appearing, and the message area (and scroll bar) expanding to match the new length. It's like a 10 second delay, or something. The second time you post the same image, it expands properly and automatically.

You only see something like this:
[icon] (9:10:52 pm) Super Saiyan X: (image)
[first 10 pixels of the image, height wise)

Note that the area is fully scrolled down, yet you can't see it.