The community members location map is done exactly like this and I did that in ImageMagick. If you have a world map in a mercatory style such that its stretched to linearize the lat/long coords or a local map small enough such that the lat/long range is approximately linear then all you need is the bounds in lat long coords of the map and the image pixel size and then you just use the ratios to go from one coordinate system to the other. If the map is in some other projection and is large enough such that increasing the lat/longs produce curved point sets then you need to do some math. It can get a bit tricky then but libraries such as GDAL can do that calculation for you.

If you have the X/Y points and you just need to plot them well, image magick does make that very easy indeed. I would suggest using a scripting language with it to make it faster such as Perl where you can get PerlMagick which is the interface of image magick with it. Then its just a loop and a few calls to get it to plot it out. There are other interfaces too such as Python etc as well. Your kinda spoiled for choice with IM.