In case you didn't read the tooltip of Settings / Collection / Preferred album art filenames (comma separated), it says:
When looking for album art, Strawberry will first look for picture files that contain one of this words. If there are no matches then it will use the largest image in the directory.
It's more than enough for me, because I use subdirectories for artwork, and there's only a front cover image with the audio files. My list of words is front,cover.
There is no Strawberry option "Only use pictures containing this words", so I'd suggest a workaround when some albums don't have a proper front cover and you don't want Strawberry to display the biggest image:
You might add something like def_image to the list of preferred names (for example, per your preferences, file,Front,cover,folder,def_image), and copy the default CD image from github (or create a custom one) to the album folder as def_image.jpg.
A bit of command line magic will copy that file to every album directory that needs it. Or you may go nuclear and copy it to every album folder, to test and delete easily all of them if needed.