I have a couple of tips for you.
1. It is a lot more effective to shrink a larger image down to the size you need, rather than trying to increase a smaller image.
2. Use a good quality graphics program like PhotoShop (some share-ware programs are equally as good) to resize your image, as they seem to keep the detail more accurate.
3. Do not be afraid to CROP your image as well as resizing it. Cropping will help you get rid of the useless and un-important areas of the image.