plotting and scaling?

I've created a small program that will do some calculations, and now I want to plot those calculations on pictures. Here's a problem to make matters worse. The scale of the images(grids) are different.

This isn't going on a map, but it's the best description I can come up with. Picture GPS coordinates being plotted on two maps. One, Illinois, and the second, just Chicago. How to I change the images so that I can draw the points and lines? AND.. how do I convert my numbers into points on the map? I imagine it's a mix of scaling, and interpolation, but I've spent a day and a half of family time searching for answers on vb.net with out enough luck to matter.