This led to a crash when a cape or skin was not stored on disk.
The function caled skins.save_image and returned that function's lwip image object instead of the expected buffer.
skins.save_image also no longer returns the image object because it's not used anywhere
this was originally implemented because we wanted to delete the oldest images on disk
where 'oldest' means not *used* for the longest time
that's not useful and was never actually implemented, so we don't need this
afaik you can have persistent storage on heroku, at least via addons
and heroku is probably not the only environment where one has a temporary file system
