It's very simple. All you need to do is copy the link to the image page and paste it into the text box.
You don't need to link it with the link button. All you will end up with is a link that just reads "link text." If you do it the way I described above, your comment will automatically have a thumbnail preview of each image link you pasted once you hit the "Post comment" button.
I think this should go into the markdown reference here https://www.girlswithmuscle.com/comments/mar...
Maybe... that would be Chainer's decision to make.