I've talked about proportion on covers and the size of your text, but spacing is just as important. You don't want things squished together or so far apart they don't even look as if they belong together. 

The image of the princess is a bit off-center. It's just enough to make me uncomfortable with how the cover appears. 

Let's not only center her but make her a little bit bigger as well. She looks much better centered on the cover and making her slightly bigger fills empty space. Since she's the main focus of the cover you want her to be front and center.

When you have oddly shaped text or images it's sometimes difficult to eyeball it and know it's all centered. In Photoshop you can add grid lines (I have mine sent to the medium blue because I'm old and blind and I can't see the default bright green ones very well). I also have my rulers set to a percentage instead of inches. My center line is at 50%, which is the middle of the canvas. I added grid lines at the edges to help see things better, you can add horizontal grid lines as well. To add a grid line, place your mouse pointer on the ruler, click and hold the mouse then drag. Let go of your mouse button when you have the line where you want it. You can move them so no worries if it lands in the wrong spot. 

Keeping your images and text centered, gives your designs a cleaner, balanced appearance.