There's 3 possibilities here.
1. Let the image display as it will. This, as you've seen, will screw up a lot of Track Info templates.
2. Force the image to a certain size. This will work well with images that are bigger than that set size, but images that are smaller will look like crap.
3. Use a script that forces large images to fit a certain size, but leave small images alone. This is, in my opinion, the best solution as it won't mess up Track Info's and it won't make smaller images look like crap.
Currently, there are only two Track Info templates that do #3. They are both (I'm about to toot my own horn here) created by me. Actually, they're just the DJ and Dreamstate templates provided by JRiver that I modified slightly. I'll give the script to anybody who wants it. In my opinion, everybody creating TrackInfos that uses coverart should use it in order to keep the nice and neat appearance of the template. Dreamstate looks really bad when confronted with a large cover art image, for instance.
You can download the templates if you like.
http://doof.webryders.com/MJ/Downloads/TrackInfos/MDJ.mjphttp://doof.webryders.com/MJ/Downloads/TrackInfos/MDS.mjp