LiraNuna posted this news:

I made a screen capture to my WIP game. press R+L in game to save a screenshot in a BMP format into a GBAMP/M3CF/SCCF/SCSD.

the capture uses a key IRQ, and i believe that a ROM loader (homebrew one like the MoonShell or Kicker) will be able to set the IRQ before executing the ROM. It uses the screen capture device and it saves to VRAM bank C (that's why you see a flicker when saving a screenshot) then save in any format you wish

More info --> http://forum.gbadev.org/viewtopic.php?p=69847#69847