You can't do it.
In the Wii U your photos can be saved to your Wii message board by going to your gallery and selecting the image you want to post, then enlarging it and press the minus button, but that's about the full extent of that. Unfortunately, you can't put images from your message board onto your computer. So as far as the Wii U goes, you're out of luck. The news doesn't get better from there. The N64 doesn't have the kind of compatibility with a computer to transfer pictures. The only way you could have pictures from snap saved to your computer is if you played it on an emulator.