When I was younger, I use to love the books Elizabeti's Doll and Beatrice's Goat. They may not "inspire justice", but the books do show what life is like for many girls in Africa who are poor and live lives that many would not even imagine. The pictures are wonderfully illustrated, and the story speaks to you and makes you pity the girls.
I'm also not sure about "inspiring justice" but Mia's Story by Michael Foreman is about a girl in Columbia who lives in poverty.Armando and the Blue Tarp School by Edith Hope Fine is a book about children living as trash-pickers in Mexico who have to make the choice between helping support their families and going to "school".I'm still not sure how well small children will understand it, but there is also Tricycle by Elisa Amado about a rich (and very young) girl who struggles to decide what to do when she sees the children living in poverty nearby steal her tricycle.
You are asking for fiction picture books, but I think most of the social justice picture books (at least the ones I'm familiar with) tend to be based on true stories... such as "Sélavi, That is Life: A Haitian Story of Hope," about the plight and resourcefulness of a group of homeless children who are victims of the political upheavals and violence in Haiti; "Si, se Puede!/Yes, We Can!" about the janitors' strike in L.A, 2000, and "Lakas and The Makibaka Hotel," inspired by real-life events involving the tenants of the Trinity Plaza Apartments in San Francisco, who, coaxed by a young Filipino-American boy, join forces to resist eviction.
i found this PDF site that has a list of the top ten picture books on poverty/ social issues, not sure whether this link still works tho http://www.freedomfromhunger.org/pdfs/readinglist_childrens.pdf