Click 
on the bottom Gallery Edit View toolbar.
Select the image you want to level. The image frame is displayed in red.
Managing Objects > Working with Galleries > Using the Gallery Edit View
Use the Auto-Level button to rotate an image so that two specified points are on the same horizontal plane.
To perform an auto-level:
Click on the bottom Gallery Edit View toolbar.
Select the image you want to level. The image frame is displayed in red.
If selected in your preferences, a screen is displayed with instructions on how to perform an auto-level. To turn off this option, select Do Not Show This Dialog in the Future, and select OK.
Select two points on the image to place on the same horizontal plane.
Click resize on the resize toolbar, to resize the rotated image to fit inside the frame.
The image is rotated, placing the selected points on the same plane.
Setting Gallery Edit View Preferences
Using the Gallery Edit View Toolbar