Picture books for adults are a strange beast. They do exist but the publishers who do them are fewer and further between. I'd focus on agents who tak work for adults and children or at least agencies that rep both. Then you can focus on the picture books for kids as your initial submission. Agents prefer you to have three complete ready to go manuscripts before you query. These would al have to be for children, so you aren't quite there yet. Also, be sure to have your work critiqued by people who know work for children. There are some dark PBs out there for kids.
Happy writing!