ADE uses the first page of the document as the thumbnail, and renders it as the thumbnail size.
Yes, I knew that. But it was not rendering the thumbnail correctly. It was cropping the image instead of resizing it. I did some other research and figured out how to fix the problem on my own. It required a code change in the html of the epub file.
I just realized my reply might have sounded a little snippy! I'm sorry. DIdn't mean it that way - just got in a hurry. Thank you for trying to help!
Not a problem with it sounding snippy. The key bit for you situation is was that it was rendering it at the thumbnail size (90x130, iirc), and as you found out it if your HTML was making some bad assumptions (usually in the size attributes for the image element) that it would be rendering in a larger viewport, that would be causing the cropping.
What I didn't know is if it was your authored content or not, because if it wasn't then the easier fix is to just replace the thumbnail image in the DE Thumbnails Directory.