The message body div is scrollable, basically.
Yes, but nitro2k01 is right - the result sucks. You get a horizontal scroll bar which covers all images so to look at one image when there are a few in the post means scrolling down the whole page, then back up.
It would be better if each image was seperately scrollable - ideally with a vertical scroll bar as well.
Originally I thought that using the [img] tag was not much better but then I discovered that you can at least control the size using that, you can also embed in-line links with an <a> tag so you can get a small image which you can click on to get the full sized one. Not sure whether you can do much better but that, at least, is better than adding the images as an attachment if you can host them somewhere yourself.