You will need some type of graphics editing software such as Jasc Paint Shop Pro, Adobe PhotoShop, Ulead Photo Impact, etc. Remember: backup your original files prior to making any changes. Open the image in your graphics software. Make sure the image is set to True Color mode. If not, increase the color mode to True Color. (All GIFs are 256 colors.) If your software requires that you add a layer before editing your image, do so now. Click on the add text button (each program has one). Be sure that you have “anti-aliased” TURNED ON or your text will be jagged. Add your text and then save your image with the original file name (eg: header.jpg).
Further Reading:
Adding Text to an Image
Adding Text to an Image: Common Mistakes