I had the same issue when I used Internet Explorer. What I would have to do it click the picture
in the post once, and the picture window would pop up, but was blank just like you said.
Then, if I closed that blank window, and clicked the picture in the post a second time, the picture
would come up fine in the picture window.
I switched to using Safari for my browser, even though I am on a PC, and when I click pictures
in posts now, they come up the first time.
It must be something in the settings of the different browsers.
Jim