Code to thumbnail payment icon image had an error for python3 because
from RPC the image would be a text string instead of an expected
bytestring.
This change accepts both and made another small fix in the view since
image_medium doesn't exist (and image is already thumbnailed).
A third fix was that removing an image caused an error.
opw-803722
closes#22088
Nicolas Lempereur
dc61861f90