This is the easiest route. Many dedicated members of the GBAtemp community have spent years creating and curating massive cover art packs specifically formatted for Snes9xGX. These are often called "Cover Collections".
: 32-bit depth with an alpha channel to maintain transparency. snes9xgx cover art
Enhancing Your SNES9xGX Experience: A Complete Guide to Cover Art on Wii This is the easiest route