Police, city staff, removal companies - all seem to have lots of negative material on graffiti, murals, and legal walls. Often it's years old, or from a very different environment, or just plain biased.
To try to balance this I've been monitoring the media online for a while, looking for a postive slant, and posting links on a few of articles in here, or on Facebook.
Previous links were collected in this post, but I have now set up a separate blog so that I can easily post in a separate space all the articles that I find. You can set up an RSS feed from it, or just check back. Or watch my Facebook newsfeed for some of them.