Audio Cover Image Size
Using Jamroom
You could try setting a Minimum Image Width in your ACP > Image Support > Global Config
/image/admin/global
I would set it higher than 256px, say maybe 300px.
You could also use the form designer on the profile create/modify sections and add in a size limit for a sub label, that would at least help let your users know there is an image size requirement.
Ultimately, it is up to your users. If they upload an image with an odd size, taller than wider or vise versa, then it is just going to get cutoff when cropped.
Hope this helps!