Actually it could still be illegal under Houston statutes which dicatate everything from mandating drab gray/brown porno bookstores (can't have a colorful mural even non-sexual) to no "billboard" or offsite advertising on the side of your business. This Obama mural probably violates the billboard ordinances of Houston.
Billboard laws are differnt than building signage laws. You keep comparing apples to oranges and that just doesn’t fly. Again, this incident happened in a suburb of Houston; not in Houston proper so the Houston laws do NOT apply.