There are two ways to ensure you keep the aspect ratio when inserting images:
1. Start to resize the image by clicking and dragging one of the corner grab handles and after you have started dragging hold down the shift key on your keyboard and the resize will snap back to the aspect ratio.
2. If you forget to hold the shift key then once you have resized just use the Uncrop button to make your image go back to its native aspect ratio.
Thanks to Steegle for this information!