What happens to my original images?

That depends.

When using ShortPixel Image Optimizer, your original images are physically replaced with their optimized versions. If you leave the Backup option active, then the originals will be saved in a separate folder, so you can restore them, should you ever need/want to do that.

When using ShortPixel Adaptive Images, your original images remain untouched, because the plugin works on-the-fly: How does ShortPixel Adaptive Images work?