I believe the usual rule of thumb for what's allowed is one image and the whole thing (any text included) shouldn't be larger than about 650 pixels. (In either height or width.) Now, that said, I think most people won't get fussed if someone pushes the pixel limit slightly or if they had two small graphics, rather than one larger one -- it really comes down to just being considerate of other people using the forum around you and not making them scroll forever or have to wait for a bunch of images to load. (Which can be troublesome on some phones, especially.)