I've thought about making a screenshot mapping tutorial once, but my tendency to do things fully and perfectly or not at all quickly shot that down.
A good tutorial would cover the basics first (taking screenshots and pasting them together), then continue with intermediate (disabling layers, transparencies) and advanced techniques (memory hacking, video buffer capture, emulator specific and custom tools, etc).
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Current projects: Bucky O'Hare (NES), Metal Storm (NES), Clock Tower (SNES), Ristar-The Shooting Star (Gen), Sonic The Hedgehog (Gen), Sonic CD (Sega CD), Mega Man Zero (GBA), Battletoads (NES)