Pixels cannot be compressed. If you discard pixels you must lose resolution. PNG compression is pretty good, especially with images with few colours, which is likely to be the case with your (Google Map?) images. However, that reduces the size of the image on disk, not in memory. To reduce the RAM needed one can either reduce colour-depth or reduce resolution; either way one loses data.
How big are the images being joined together? If they are a typical screen shot, they may well be more than 1 Megapixel.
Perhaps someone will pass by who is good at writing batch files, but first let's discover if your project is even feasible.