You don't have to find the Black Widow. She will find you. (Check the colorful painting.)
in the inspectors cottage
black widow
You cannot locate it. In the course of the game, it will be given to you.
The Black Widow is never in the museum (at least not as the Black Widow).
Use the key given to you by the museum curator, and enter the cottage. Peel back the corner of the painting, and you will get captured by the Black Widow, and taken to her underground lair.
skrow you
After you chase the thief on the scooters, you will get the key to the cottage in the Country Side from Early Poptropica island. There you will find the missing painting and enter the lair of the Black Widow.
The Black Widow (veuve noire) is posing as the police inspector, whose house is in the Country Side. It is on the far right of Counterfeit Island.
The Black Widow is behind the theft, and after you chase the thief on the scooters, you will receive a package that leads you to her. (You will get the sought after key to the cottage in the Country Side.)
the black widow
you can't
beat the black widow