To flip an image, icon or any graphic element, first click to select it. Next click on the Flip icons in the upper menu. You can choose to flip it vertically (top/bottom) or horizontally (left/right).
Undo individual actions by using the Undo/Redo buttons at the menu toolbar on top. Or use keyboard shortcuts CTRL-Z to undo and CTRL-Y to redo.
To reset to initial orientation, just double-click the rotate handler (the white circle at the top of the element). Resetting will return a graphic element to its original size and rotation.