I looked at this high res version of the photo(click for close up) close up and I(not being an expert)don't see that it's been photoshopped. However after a close inspection a couple of things are clear. The offending word isn't taped on there, it appears to have been set in front of the sign. The most obvious thing is that the handwriting on the offending 'n' word does not match the handwriting on the sign above.
Take a look at the A and the R they appear multiple times in the sign above. the right hand leg of the A in the bottom word has a sweeping inward bow that does not appear on the other 'A's in the sign. The 'R' also has a distinct outward bow in the right leg with a pointed foot. The other two 'R's appearing on the sign are not even close to the 'R' at the bottom. So, I don't really know what is going on here, but I can tell you that I smell something fishy.