You might look into sprite/tile sets, instead of the rom/screencap method. I know Brian from 8-Bit theater uses a sprite set from Final Fantasy which has every sprite, including alternates for the 2-3 frame "animations" used to create the walk sequencing, etc. All characters, classes, npc's, mobs, bosses, weapons/equipment, interface elements, etc.
I also think David from Bob & George does the same thing, only uses multiple sprite sets for the various Mega Man games.
Basically the sprite sets are created by someone pulling the sprites/tiles from the rom files themselves. They're usually tiny. Like NES sprites/tiles are generally 16x16 pixels up to 48x48 pixels for some of the larger bosses.
If there aren't sets available for the games you're using, you might look for level editors for the game system itself, which should let you extract the sprites/tiles from the game rom's yourself.
Graphic Design / Photography / Web Design