I just ran across this and solved it, but wanted to post in the event anyone else runs into it. I have a jpg file of an industrial Joystick that is much taller than it is wide. I had been setting my pop ups to 1000 px for desktop applications and when I set this image to the same there was no + appearing on the cursor when hovering and it would not pop up while being viewed. I changed to 500 px and all is well. It looks like if an image is going to exceed the display size it just ignores the pop up setting.

Ed