I solved this by putting the image into an IMG tag within the raw format, i.e.
post.raw = "<img src='https://cdck-file-uploads-global.s3.dualstack.us-west-2.amazonaws.com/meta/optimized/3X/9/d/9d543e92b15b06924249654667a81441a55867eb_1_690x184.png'/>\n\n"
post.save
..means that it cooks on the save fine now.