If that was a CocaCola mural or Britney Spears album mural, it would still be regulated.
"Billboard" technically refers to offsite advertising (which is why Alabama Bookstop advertising a show at Alley Theater on their marque qualifies as a billboard). The yellow building in question is a business but it also has served as a campaign office at times. Unclear just what may or may not be regulated.
http://www.publicworks.houstontx.gov/planning/enforcement/signadmin.htm
http://documents.publicworks.houstontx.gov/document-center/sign-administration/sign-code/details.htm
SIGN shall mean any outdoor display, design, pictorial or other representation that shall be so constructed, placed, attached, painted, erected, fastened or manufactured in any manner whatsoever so that the same shall be used for advertising. The term "sign" shall include the sign structure. Every sign shall be classified and conform to the requirements of each of such classifications set forth in this chapter.
SECTION 4619--ABATEMENT OF OFF-PREMISE SIGNS
(c) Exclusion. The provisions of this section shall not be construed to require the removal of a structure that is used exclusively and at all times (except when there is no copy at all on the structure) for messages that do not constitute advertising, including, but not limited to, political messages, religious or church related messages, public service, governmental and ideological messages and other copy of a nature that is not commercial advertising because such a structure is not a "sign" (either on-premise or off-premise), as that term is defined, for purposes of this chapter and is not subject to regulation under this chapter.
--- The size of the sign and the nature of the building may play a role. But it may also get a pass since it went from being an AD for the candidacy of Barack to a tribute to the President. Only it wasn't a tribute the office of president. It was a tribute to Barack Obama and remains such. If Alan Keyes or Michael Williams wins the White House in 2012, don't expect a mural honoring either of these black presidents on that wall.
If the yellow building is a private business, then they may still be legally able to put the mural on their building.
If you notice in Houston, the zoning laws & city ordinances are very liberal and rarely enforced due to low manpower in their law enforcement areas.
This building may be skirting the law due to the phrase “used exclusively and at all times (except when there is no copy at all on the structure) for messages that do not constitute advertising, including, but not limited to, political...messages. If the building is not used exclusively for these political messages then this building may be except.