Can anyone advise me on increasing pixellation on low resolution pictures?

[InTheCludgie]

I have some pictures which I'm hoping to sell copies of on a print on demand service however the resolution on my 20 megapixel camera isn't high enough for most sizes of wall art which is limiting the amount of items I could potentially sell.

I've noticed that the 3d paint option in Windows has an option to adjust resolution on pictures, does anyone have experience of using this? Is it effective or should I just get these professionally scanned or photographed? Thanks!