You'd need to take a video output from the machine and put it into a Video capture card, then grab a still using its software or something like Photoshop or Paint Shop Pro. A cheap card will only have composite or s-vid in so you'd need one of those.
Frankly, with a cheap card and the conversion processes (RGB>whatever, then the capture card process), the images will be pretty shite. I know I'll be frowned on for saying it, but this is the thing an emulator is good for - screen shots. Just use an emulator!