I had been doing just that, but with the image in the viewer window. I tried doing it with the image thumbnail window. Command-C and Command-V. Nothing is pasted. Then I right-clicked, selected Edit with Grab, copy and pasted from Grab. Pictures appear in the new thread box, but when posted, they are gone.
For anyone that is serious about maintaining control and having a good longterm backup for their photos, I highly recommend a separate hosting service. There have been some free ones but they've all proven unreliable in the longterm Photobucket is one recent example, after years of allowing impeded photos Photobucket changed its corporate mind and broke thousands of links in forums across the web (probably more like millions of links) unless each user gave them money.
I use SmugMug to host my photos. Hosting photos and videos is their only business. It's not free, but the basic account is only $3.99/mo (when paid yearly). Here's a breakdown of SmugMug's pricing levels:
But getting the credit is NOT the reason I'm recommending SmugMug. SmugMug is an awesome photo/video hosting service. Although I had some issues with the service a few years ago, it has proven extremely stable and user friendly since then. I'll have a SmugMug account until the day I'm gone.