You just need a graphic program to fiddle with image files.
As mentioned, graphic programs range from simple to complex, free to expensive.
GIMP is a commonly used free one these days.
Edit: Beaten to the punch.
If you don't feel like spending $650USD for Photoshop, PaintShopPro is a fine program for about $100-150. It's not quite as featured as PS, but it should fit most of your needs for things like signatures, as well as a lot of other things.
2ndEdit: I believe going around the trial-version limitations would also be illegal, so let's not go there.