Should I have been fired for playing this innocent prank?

I started working in a famous high street home ware store about five months ago. A colleague and I were bored one day, so we decided to put pictures of page 3 topless models in the empty display frames in the home furnishing department. The shop got complaints, but they didn't know who did it. However, they looked at CCTV footage of the stock room and saw me and my friend putting them in. It was only an innocent prank, but now we've both been fired. Do you think that's fair?