Go to your PB account find the picture you want to post, select the link button at the top of the picture, it's what looks like to chain links, it will show 4 links, you want to copy the IMG link and paste it in your post, use the advanced post so you can see the picture before you submit the post. If it doesn't show up you might not have copied the whole link. It's actually quite simple once you do it. This picture is from your PB, I did not resize it.
