Instructions
Go to the Museum on Counterfeit Island, then use the X-Ray Machine right next to the painting of Van Gogh's "Starry Night." The message tells you to go to Early Poptropica Island, so go to the travel balloon, open the map and click on "Early Poptropica Island."
Walk left into the PopArt Museum on Early Poptropica Island. Talk to the woman in the gallery and she will give you a key, now that you know about the secret message. Return to Counterfeit Island.
Go past the museum into the countryside on Counterfeit Island. Use the key on the Inspector's House, then head to the top floor to examine the painting there. The lights will cut off and you'll be knocked out and dragged away to Black Widow's Lair for the final act on Counterfeit Island.