A guy name Gabriel Pickard has been putting a lot of VTT stuff up on RPGNow lately, and one of his latest is a collection of just over 200 images for system mapping. It's aimed at VTT, but the images are fairly high resolution (planets seem to be 700x700 pixels, suns a little bigger). In the sample below, I used the default settings for the Annual Highspace System, and had to reduce most of the images by 90% to make them fit on the map. At $5.49, it's useful if you want more variety in your system maps.

Vile Tiles System Mapper

(Note: personal use license only.)

Sample of what it all looks like (there are a bunch of planets of various types, asteroid fields, individual asteroids, cloud cover overlays for planets, ice cap overlays, a comet, some circles for showing orbits that don't scale all that well, rings, and some backgrounds. This is a very small sample.