Sirius had the newspaper cutting with a picture of the Weasley family in Azkaban. He then swam back to mainland as a dog.
So, one night when they opened my door to bring food, I slipped past them as a dog... It's so much harder for them to sense animal emotions that they were confused... I was thin, very thin...thin enough to slip through the bars...I swam as a dog back to the mainland ... I journeyed north and slipped into the Hogwarts grounds as a dog.
But in the Shrieking Shack, he pulled out the Weasley family picture from within his robes. How did Sirius still have the picture if he had to transform into a dog to escape?
Black put one of his claw-like hands inside his robes and took out a crumpled piece of paper, which he smoothed flat and held out to show the others.
It was the photograph of Ron and his family that had appeared in the Daily Prophet the previous summer, and there, on Ron's shoulder, was Scabbers.