When adding your logo to your photos, it is important to remember that depending on the print size your customer chooses, the logo has a chance of being cropped off.

If you use a SLR or Digital SLR, a 4×6, 8×12, 10×15, 12×18, 20×30, and 24×36 are all considered full frame and uncropped.
If you use a small digital Point and Shoot camera, most wallets, 4.5×6, 8×6, and 18×24 are considered full frame and uncropped.

If your customer picks a size other than the size listed above, then their print will be cropped and your logo maybe cut off if not properly placed.

For a SLR, place your logo at least 1/6th or 17% away from the short edge of the photo. Your preference to the left or right side.
For a small digital Point and Shoot, You can place your logo closer to the short edge, but move the logo inward away from the long edge.

*Keep your logo small in size and add a drop shadow so it will stand out on different backgrounds.

If you use Adobe Photoshop, then you can place this logo on your images and batch process a whole set of photos with your logo.