Each time i check a thread map, i open the pic in a different tab so i can see what the map is alike, and if i already know the map to have a fresh view about it. Wouldn't be nice to have the picture directly in the thread? I guess i am not the only one.
If a pic is too big, at least a (rather big - like the minimap) thumbnail so you can at least preview it without the need of opening a new tab/window.
The maximum image size in mapthreads would be 600 (width), more than that kills the page's design (you know, having to scroll horizontally...). But you would need at least that size to get an acceptable view of the map.
It also increases loading time and forces you to scroll down before you can see the comments.
So, these reasons would certainly not make it impossible to have pics in the map threads.
I personally wouldn't like it, dunno about the others, but this is the first time I hear such an request...
Anyways, you can always add a picture in the mapthread with those html tags, linking to the "normal" map picture.