Just tested the upload from the gallery Dennis and I had a couple of problems first but managed to do it.
I think the problem you and Meg are having is that you ar selecting the URL for the thumbnail sized image. You need to dble click to get the full size and then copy the full size URL as below. I got the same result as you if I copied the small image url.
I was successful after opening in chrome, going into my gallery, clicking edit at the bottom of the small cover photo. This took me to a page with the 4 pics I have on my gallery one above the other or side by side. Double clicked on the image I wanted and right clicked once which gave me copy image url. Contol C copied it.
On my forum post select the text or HTML tab from the far RHSide.
A box opens with http/: blued out ……. DELETE this and do a CONTROL P which copies the URL into the box.
Click on Visual tab (far RHside) and your image will appear.
Click submit. That is it.
Hope that helps. (sorry this pic is a little bigger than normal)
I would advise playing around until you see the full size image in the post when you finally click the visual tab after copying and pasting as once you have submitted your editing tries are restricted. What you get in the visual tab window will be exactly the same as after submiting so if it isn’t right before it wont be right after.