Game is 100% playable, but an error message keeps popping up saying Ninja_00.png is missing...
Heyho JonIrenicus!
Thank you for playing and for reporting the bug! 
I guess you're on Linux?
Sadly, we won't be able to fix this error right now due to the IMGC2017 contest rules (we're not allowed to upload newer versions as this would skew the competition). After this, one of the first things on our list!
Maybe this hotfix can help you play (quoting user DrkSky from below in the comments) :
"so ninja_0X.png files are being called improperly and throwing a load error.
So is the LetterSound_Neutral.ogg.
GNU/*nix players can manually rename the files changing the case as required, untill a fix is released. A quick enough rename."
Stay funky!
Dez