If you do a search on this forum, you'll probably find extremely detailed information on this in older posts--but I'll give you my general understanding:
Emotes are limited to a maximum height (for some reason I think it's 50px--but don't quote me on that). Any emote/gif that is uploaded with a height greater than this maximum height will be scaled down and will only be visible at its original larger size by clicking it and having it pop up in a larger window. So if you make your image fit within that height restriction, I'm pretty sure the width can be significantly longer, thus the many existing long banner like images on MFC.
There are some emotes/gifs/images/whatevers that are larger than the height restriction, but I'm pretty sure these were submitted before MFC implemented the height restriction. So as far as I know, there is no way to get around the guideline--you just gotta make your image fit that 50px restriction, or just hope that people are willing to click. :-D