Am I incorrectly converting the imgkit image to bytes or is there some other error?
I would start from checking if your bytes represents image understand by your Pillow. Built-in module imghdr should suffice if you are excepting one of format known by it (see table in docs). Usage in this case:
import imghdr
...
print(imghdr.what(None, h=img))
If it does identify format then check if it is supported by your Pillow, else you would need to manually check file signature (few starting bytes).
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