The DPI values is irrelevant. What counts is the resolution of the image when it was taken. If you take that image and print it on a small size of paper, it would have larger DPI. If you print it poster-sized, it would have low DPI.
SuperShot can take images up to 8K. It's simply the limit of what the engine allows. For more someone would need to implement a tiling-renderer, which I doubt will ever happen for VaM 1.x.