I investigated a bit and specifically Ren'Py now returns `True` for `renpy.loadable("")` in v8.4, which appears to be an engine bug. I have reported it in the Ren'Py GitHub, and in the meantime you can change the line elif renpy.loadable(s): in the function check_bg around line 124 in image_tint_tool.rpy to elif s and renpy.loadable(s): and this should fix the problem. I will be putting out this fix as an update soon.