Burned photos to dvd - they printed ...

I have many photos on my mac that I need to print and back up. I have my library backed up on an external hard drive but I would feel better to have them on cd's as well. I burned a dvd of some photos ( I didn't have a cd) . I took it to the store to print. The photos came out a little blurry. I printed some of the same ones directly off the mac and they came out sharp. What did I do wrong?
Any suggestions for backing up my photos in addition to the external hard drive?

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