No, images aren't resized automatically. In fact, it has no way of knowing what skins you will be using, so it couldn't know what size to resize to. You can hack this feature in. Here is a thread that may help.

http://forums.b2evolution.net/viewtopic.php?t=1804

netmax wrote:

But i discovered a big problem: When uploading, referencing or posting (by MMS) pictures which are bigger than the area of the skin (moonrise), the page is completely messed up by the too large picture.
I really won't believe that such a great software won't resize the pictures and messes up everything.
Marco

Have the same problem ,with the big MMS pictures .
Easyst going is not to send the big MMS pictures for me
But ther most be an better way to show the <img with some code that it shows up on a max size .
Without resizing !
Hope i was understandable ...

If some could be of help .
Thanks