There are two ways to maintain the original aspect ratio of your images:
Lock aspect ratio
Fit to container
Using Lock aspect ratio
To maintain your image's aspect ratio in your design, you can select it and toggle the Lock aspect ratio on the right side panel. This will force the container (either in a grid, or single column) to match the aspect ratio of your image exactly.
Using Fit to Container
Otherwise, if you'd like to keep the aspect ratio of your image without forcing the size of the container, select Fit to Container on the right side panel. This will enable your image to fit inside the container without clipping.