If you don't have a graphics program, you can use MSPaint. Just open the picture in Paint, then click on Image, Stretch/Skew.
Then select a number for percent, like 200, for Horizontal and Vertical. This should double the size of your picture.
Try other percentages, but keep in mind, most pictures don't hold up well when enlarging.
HOW DO I ENLARG MINI PHOTO

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic