Using Brave, tried on mobile and PC. haven't run into this prior to yesterday, but will try a diff browser later
Good to also know the format (gif, jpeg, png, ...) and file sizes.
I never upload to the forum. I upload to a free image storage site (cloud account) like imgur.com, and post the image's associated link. That not only uploads easier for me, I can edit the file size, crop the image and perform other changes. When I paste the link, instead of a thumbnail inside my post, I get the actual image displayed inline with my comment.
This also helps prevent the forum's server from hitting its storage limit with a bunch of large files uploaded, which is one cause for being unable to post images.