Nah there's really no tutorial. I was hoping that releasing my stuff would help people out a bit though, and I'll be happy to explain anything I'm doing in my programs.

If it's the GD library you want docs for, those are available at http://www.boutell.com/gd/.

As for how to do the Hijack stuff, my source should be a good starting point. Basically all Hijack programs are one big while infinite loop that has a EMPEG_HIJACK_WAITMENU ioctl inside. This basically sleeps the program until the user goes into the Hijack menu and selects your app from the menu. Then you usually want to bind to some buttons using the EMPEG_HIJACK_BINDBUTTONS ioctl, and do your thing. When you want to return to the Empeg's player app, you just do a WAITMENU again (usually by letting the outer while loop run again.)

emptriv isn't the best one to start out trying to understand.. empan is probably a little simpler. Start with that one...