The Summer 2018 billboard was a painting by transgender political prisoner Marius Mason, a former Bloomington resident, and currently a transgender political prisoner. A prolific painter and environmental activist, Mason depicted this vaquita- an endangered animal with less than 15 left worldwide, as of March 2018.
You can find out more about Marius Mason here: https://supportmariusmason.org/
You can find out more about the vaquita here: https://defenders.org/vaquita/basic-facts