Digital billboards are called “image-changing” signs, but they call so much attention to themselves they can change the image of a city neighborhood as well.

The City Council, then, which already is working to update its billboard ordinance, should move cautiously when establishing the ground rules for these new glowing signboards. The signs could bring much-needed revenue to the city but also could intensify visual blight.

Mayor Rahm Emanuel has cut a 20-year deal to put up 34 digital signs on city property adjacent to the Kennedy, Dan Ryan, Stevenson and Eisenhower expressways and the Chicago Skyway and Illinois Tollway. It would guarantee taxpayers $15 million in 2013, going a long way toward meeting next year’s budget goal of $18 million in municipal advertising revenues.

While he’s at it, Emanuel also wants to allow digital billboards up to 100 feet tall on any of 12,000 city-owned properties throughout Chicago. The city could get a cut of the revenue, he said, and for each image-changing billboard that goes up, five conventional signs would have to come down.

The administration would like the City Council to act on the digital billboard plan next month. But when studying Emanuel’s plan, the Council should look beyond the dollar signs. Ugly billboards have a way of sprouting like weeds.

It would be unfortunate if the Council’s effort to eliminate illegal, hideous and unused conventional billboards simply made room for pervasive new electronic imagery that some Chicagoans already hang up blankets over their windows to screen out, even from 2,000 feet away.