Just run iPrint Photo and select an image. Then watch it come up on a screen to show you what's going to be printed. Check that your printer has been "seen" by the app near the bottom of the screen and hit the "Print" button to have the printer spit out a 4 x 6" print. That's about it - although you CAN have it check for other printers on the network to select the one you prefer to work with (a bxw laser versus a color inkjet, for example).

click to enlarge
And sure it's limited to the pics being on the Touch. But it's easy enough to get images into the Touch from other sources if you put your mind to it (helped along by other apps, like MOVER which lets a Touch send images to another Touch over a wireless network).