Pixelated background image

Hi,
Im having problems putting a background image on my site. Not only that but any theme i install brings the same problem.
Im a complete amateur so this is my first site. However, iv tried enlarging the pic, changing file type, chosen an image with a higher resolution, tried using a photo, im at a loss!
Any image i try to use for the background, or even just a colour, looks pixelated, like hundreds of little squares.
I use a netbook but iv checked on a normal laptop, pc, and on my dads internet on the tv, same issue!

Can anyone help me please? Thank you!