Annnnd problem solved! Sorry for the mess, everyone, but this was terrible to sort out. It was so much easier in the good old days of photobucket and everybody's devices having a nicely low resolution that fits an ancient forum like the 'downs...
A recommendation for fellow mods in the 2020s:
1. The sooner you accept you will have to manually* resize every image you get, the easier your life is. (* I actually ran the files through a resizing website as a bulk upload, but still. I made everything 800px wide, but perhaps even less would have been better, at least for the vertical ones.)
2. Google is evil. Drive and photos don't work with the forum code. Use Imgur.