Go into photoshop, or any basic image editor and increase the size. You may have to stretch or even squish the image which may or may not pixelate it...
EDIT: Where as the image is square it shouldn't squish it, which is good. But you may still notice the pixelation from stretching it.
The directions using MS Paint: (Because I don't have Photoshop or anything nice on this computer)
Load your image.
Image >> Attributes - Set width and height to 1024
Select the entire image: Ctrl-A
Image >> Stretch: 200%
Save