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.
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.