Precondition:
1) 1th category: files which names include "front" or "cover" (high priority)
2) 2nd category: files which names include "disk" or "back", "booklet", "cd" (lowest priority)

Algorithm:
1) get all images from current directory
2) check aspect ratio (can be > 0.8 and < 1.2)
3) if found file with high priority - this is it. Stop.
4) check subdirs .cover (case insensitive) using same algorithm. If found file - use it. Stop;
5) if exists file not from 1th and not from 2nd categories - this is it. Stop;
6) use first of rest files. Stop;
7) check ~/.covers directory (covers downloaded via TPlayer). If found - use it. Stop
8) try to extract cover art from music file using TagLib
9) return default cover;

If you know C++, check this to understand algorithm: https://github.com/niktischenko/some...overfinder.cpp