Let me explain the whole situation.
I had a recent customer of mine call me up and ask if I would give him permission to "blow up" a 4x6 print into a 20x30 print. If I agreed I needed to contact his local Walgreens photo center so they would release the print to my customer.
He scanned a 4x6 print I sold to him and uploaded the digital image to Walgreens web site. Then the Walgreens lab (off site lab) print the 20x30 and shipped it to the local Walgreens store. Along with the print the off site lab informed the Walgreens employees NOT to release the 20x30 print until my customer provided Walgreens with a copright release form signed by me.
Of course I did not give him permission. We also tried to work out something with this customer to purchase a 20x30 print from us with a discount since he has purchase from us before, but he has not replied to our phone calls or emails. Honestly I don't think he will.
Now to my question. How common is it for photographers to add a small copyright logo to one of the coners of the photo so that it does not interfere with the image? We are also think about printing the same copyright info on the back of the prints. My print lab offers this service.