If you launch the Image Capture program and pop into its preferences and set it to do nothing when a camera is attached, that should stop iPhoto from starting each time you plug your iPhone in.
Maybe in iTunes if you tell it to manually sync your iPhone that'll stop it from popping up.
Just like photos on the iPhone, screenshots on the iPod touch are stored in the Photos app, though in an album called Saved Photos (instead of Camera Roll on the iPhone). Whenever an item exists in Saved Photos, plugging the iPod touch into the Mac causes iPhoto to launch, since it assumes you want to copy the image to your Mac for safekeeping.
The solution is easy. Open the Photos app on the iPod touch, tap the Saved Photos album, tap the screenshot, tap the trash button in the lower right corner, and tap the big red Delete Photo button that appears.