Before I log in, I have noticed that no matter what I do, most large images posted (those that stretch far beyond my screen's max size) stay this large and the little rotating "busy" arrow keeps rotating on the new tab.

After I log in and refresh the page, the rotation stops and the image shrinks to fit the allotted size of the page. Anyone else noticed this? Tried searching the forum, but couldn't find anything about this being posted before. Maybe just unlucky.

I use Firefox and I am not having this problem.

Could be an older browser issue. I am in the middle of upgrading everything to latest Mint 20. Got 19.3 downloaded and dd'd onto a USB stick when news were out of version 20 coming out shortly. So had to wait and start over.
I have things to do besides computing, so this takes longer than expected. In the mean time, these things might become even more common. Oh, well.

Thanks.

IIRC there are no thumbnails on GC. Your browser is always served the full-size image, and its is inserted in HTML code that resizes it. But the browser cannot resize it until it is fully downloaded, so you can get a bit image temporarily.