Because Web server pricing is determined by disk space used, among other things. Alex would rather the space he is paying for go toward hosting aaroads.com than letting other people decide to how to use it by uploading whatever they want to it.
As mentioned before, imgur is a perfectly good hosting solution. Or if it's a road photo you took yourself, consider licensing it freely and upload it to Wikimedia Commons. There, it could be used by a roadgeek to potentially illustrate a road article.