that are being displayed at 100% when the thumbnail is clicked
I tried various ways to get thumbnail-links in the post that will transfer the user to imgur that shows the image 100%.
Some phpBB forums (maybe all?) have an option to copy paste so-called "Linked BBCode" into the forum post, with medium or small sized thumbnails.
Obviously this will avoid having full sized images in the post, which may lead to ridiculously large posts, specifically when posting multiple images.