Without getting into the mathematics of it (because I don't entirely understand them myself :P), there are imagine "enlargement" programs out there that use the mathematics's in generating fractals (geometric patterns based on mathematical rules) to upsize these images for large display without the typical quality loss you'd get by just doing it photoshop.
So the photo starts typically as say an 11MP jpg (maybe 4mb) but then gets upsampled so advertisers can display it as an A1 print of whatever if they want. This does need pretty decent picture quality (ie - good glass and a good body) to begin with to end up with a reasonable end product though.
Note that most microstock agencies will not allow you to do this, but have far far smaller image requirements (>4mb) to begin with. If you're shooting with a P+S, then this is a far better option for you, just be aware of (seemingly insane) strict noise and artifact requirements.
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