Thick paper with a nice pattern or in a coordinating color will work beautifully as well. If you don't trust your cutting skills, go with a back matted picture.This is when you quite simply place the picture over the card stock or paper and adhere with a bit of double stick tape. This holds the picture in place and still gives it a nice surround.
Martha Stewart makes a package of 12x12 papers for scrap booking that costs $6 at WalMart and comes with 36 sheets of thick, card stock like paper in a variety of colors. This is perfect if you have quite a few pictures that need framing and they are less than 12". Stores like Michael's sell pre-cut matting in fairly standard sizes, but this will cost a bit more. Keep in mind that as long as you have a nice even edge framing out your picture, it will look great