you can't really enlarge em. Best thing to do is to get a good scanner. Stretching it will make it pixelated. But if you still want to, download photoshop and you can stretch images. First stretch out the size of the pallet, in the corner box of the picture. Then hit ctrl + a (select all) and I think it's ctrl+ t to open the box to stretch the image. Hold shift while you drag to make it stretch the image both ways at the same time making it uniform.
You can make images appear bigger in your posts here by adding the image in, then hovering over the image. There will be a little pencil, click that and then select large or full