Art,
You can do it in Photo shop which is a better program but I used basic old "Paint" which is a standard windows program listed under accessories. Open your photo in paint, click on the big capital A in the tool bar on the left, choose how you want it to appear IE picture intact or in a colored block of its own. I click on the lower box which leaves the photo intact and all you see is the letters. Then you create a box where you want the letters to appear on the photo that's also big enough to hold your font size, and type away. The hard part is figuring out what color will show up best on the background after you save and post it.