WOW you guys were an AMAZing help!
Yes, Johan and Hans, the filename was too long so it didn't show up. I guess it wasn't the album art.
The other file was a live recording at a sample rate of 8kHz, so I opened it in Audacity, resampled it at 44.1 kHz and exported it as per the specs in the link Fred posted.
Both were displayed and played on the Yamaha as regular.
Thanks for all the help!