Most picture books will be listed in WorldCat but there are a lot of different ways they get listed. Among the possibilities:
but the subject phrase with the most listings is "pictorial works"(61,989)
Please note that many of the books which other sources recommend as excellent picture books are not designated with any of these phrases in WorldCat, I believe because they actually have some words.
Here's a few pages to check out:
Children's Picture Book Database Miami University offers this search base of over 5,000 picture books: "a place for designing literature-based thematic units for all subjects."
The Reading Tub is a nonprofit organization that promotes reading. Their Book Reviews section includes a good number of picture books, but there is no way to search by theme, unless you want books on adoption, the only type they pull out.
100 Picture Books Everyone Should Know a list of picture book recommendations from New York Public Library.