I absolutely despise WMP for not only littering my folders with those crappy resolution AlbumArtSmall.jpg files... but much more egregiously, overwriting carefully selected folder.jpg files of my own choosing with a lower res (and sometimes just blatantly WRONG) folder.jpg of it's own choosing.
</end_rant>
...how do you know for sure that Opus is choosing the AlbumArtSmall.jpg file? You can try just setting the Preferences / Miscellaneous / Advanced: cd_thumb_coverart_file explicitly to folder.jpg - which is what I've done myself... but even disabling that doesn't result in Opus choosing the AlbumArtSmall.jpg file instead of folder.jpg. Are you seeing a regular folder image thumbnail icon, or the CD cover overlayed with the image as the icon?