You can specify the aspect ratio for media files using the buttons located next to each item in the playlist window.
You can select the desired aspect ratio from the drop-down list. The Auto option will leave the original aspect ratio of the frame, and other options will stretch the image to ensure the desired proportions.
In the Cacher mode, you can adjust the image aspect ratio by cropping the edges of the image at the top/bottom or on each side. You can do this using the corresponding button on the toolbar at the bottom of the viewer.
The required aspect ratio is also selected from the drop-down list.