Things are even worse: it's not enough to know the pixels/meters ratio. You have to know a) which projection was used and b) its exact parameters. It seems obvious that a pixel near the North Pole on a rectangular map represents a larger e-w distance than a pixel on the equator.

I'd read http://en.wikipedia.org/wiki/Map_projection if I were you.

BTW, you can compute the east-west distance represented by 1 pixel on the equator easily, since you know the length of the equator and its length in pixels on your map. That will give you scale if you find which projection was used.