Smartphones and Search algorithms are almost certainly developed enough to look for a webpage without requiring an ugly blob in the corner of an advert or information pic.
Just sayin'.
arguably not an idea.
okay fine if you insist.
The idea is to duct-tape together Google Images (or similar)
on the web-end and character/pattern recognition software on the phone end.
The advertising company puts its ad picture on its website a few days before releasing it into the real world, along with the keywords that are on the ad: that gives Google/other enough time to pick up on it.
A few days later when the ad is entrenched in Google images along with its keywords, take a picture of the ad, your text recognition software grabs the text from the billboard and does an image search then does a rough comparison of the results to find the match which is the webpage the ad wants you to go to.
Livin' the dream.