I wasn't even aware MC had the ability to get posters from the Internet. How does that work?
I use Person Video Database to collect meta data and cover art (imported to MC using PvdImport). I don't expect everyone to go to the same lengths to get such data, but my experience suggests it's unlikely there's anything MC can do to improve the quality and consistency of posters. I use PVD to get the best available posters using several different sources. The better ones are not likely available to whatever means MC is using. Even then, a manual search is sometimes required to find an appropriate poster for more obscure videos.
Perhaps the best way of dealing with this, short of using PVD/PvdImport or MyMovies, is to set up search links for some good poster sites. Then, if the automated method fails, it's not so difficult to find one via a search. Having found one, all you need to do is copy the image or URL, and then use Cover Art - Paste from Clipboard.