While the accepted answer works, it certainly is very tedious for a large amount of objects, such as in this case, since you have to edit a lot of parameters. Thus, a better solution would be as follows:
The components are fairly straight forward. The first Math node is just putting the input random value in the range from 0 to n; then the other two after it are a floor rounding. Then this value is used in a MixRGB node to decide which image to use - if the Fac is 0, it uses the top image, otherwise it uses the bottom image (since it can only be a integer, there's never any interpolation between different images). Then, if there still are images to choose from, what you can do is subtract one from the output of the floor function and use that as the Fac between the previous mix and another image. Then you can use the Random input, found in Input>Object Info>Random to get a random texture.
It is fairly easy to expand this setup to allow more images:
And you can use the same process to expand it even further. Just remember to go out of the group and update the n input too.
Here's a quick demonstration of how it works:
http://blend-exchange.giantcowfilms.com/embedImage.png?bid=325
