You just open your image in an image editor and resize it to the dimensions you want. Photoshop seems to be the choice of champions for just about anything graphics-wise. You can do something like this in MSPaint, but it's a pain in the butt. If you want something far better than MSPaint (Assuming you're using Windows, if not
Seashore for Mac is similar), but not as steep price-wise as PhotoShop, I suggest downloading
GIMP. It's free, and there are loads of tutorials for it out there.