New York and Boston.
"Until the pictures of the slave's sufferings were drawn and held up to the public gaze," said Angelina Grimke in 1838, "no Northerner had any idea of the cruelty of the system, it never entered their minds that such abominations could exist in Christian, Republican America."
A major means of publicizing such images were the Almanacs that the American Anti-Slavery Society began bringing out annually in 1836. The illustrations from 3 of those are available here: 1838 (which focuses on the horrors of slavery in the South); 1839 (which focuses on the sufferings of free people and abolitionists in the North); and 1843. The first two are from The John Hay Library, Brown University; the third, from The Clifton Waller Barrett Collection, University of Virginia.