Err 13 permission denied when trying to save screenshot

I switched to Mac OS Sierra from El Capitan and this error appeared:

IOError ( [Errno 13] Permission denied: '/Users/macuser/Dropbox/sikuli/images/instructions.png' )

Here is the relevant code:

reg = Region(Region(278,216,390,119))
img = capture(reg)
shutil.move(img, "/Users/macuser/Dropbox/sikuli/images/prompt.png")

I chmod'd 777 the images folder, but still nothing.

Thanks!