If it's a BMP, it's propably automatically converted, because for ID3 only JPG and PNG are allowed.
Maybe the album art is either stored as an upsupported image type (MortPlayer only uses "front cover" and "other") or format (technically, it's possible to store e.g. GIF or BMP, altought the standard doesn't support it...).
If you want to, just mail an example to
mort@sto-helit.de, I'll check what's wrong with it...