Under your pm reply box you are pming in - in the second box with a white boarder you will see MANAGE ATTACHEMENTS - click on that and post your pictures as you would then on a thread or post.
hope this helps
When I'm trying to drag down from photo collection I've uploaded to the bottom of "manage attachments " window to attach to post, what I have noticed that more than 75% of the time the photos just won't stick in bottom window, even after repeatedly selecting the ones I want & dragging them down, so I'm unable to post them.
I recently learned that when that happens if I close "manage attachments" window (x out of it) & then reopen "manage attachments" - PRESTO- the photos are there in lower portion of the window ready to post. Weird, but it has consistently worked for me & changed a very frustrating experience into just a couple of extra steps that don't take much time and work consistently.
I wondered if this same experience is what has lead to so many posts regarding problems re posting pictures. If the mods want, I could do a screen video shot of how photo uploading/posting process doesn't work as expected.