1. Wrong forum. Please use 'jQuery Mobile" forum.
2. What's an "image popup"? jQuery Mobile has no such thing. It has popups. You can put any kind of content you'd like inside a popup. Do you mean you are placing an image in a popup?
If you're following on of the examples shown in the demo for popups, it would be useful to tell us exactly which one you are following.
What centering are you referring to? Centering your your image within the popup? Or centering of the popup itself?
Centering of images both horizontal and vertical is a classic "hard problem" for HTML. Look at HTML5 flex-box which now deals with this easily if this is what you are having trouble with.
It's best to show a picture. (In the right forum... I realize this forum name is confusing.)