The aspect of your photos can be modified using the Scale options from the Settings toolbar. The default option is the Scale Crop mode and that will scale the picture to nicely fit the container while the Exact Fit will do the same thing but without scaling the actual image.
Maintain Aspect will keep the original aspect ratio even if the image container is using a different ratio and the No scale option will display the original image on any aspect ratio size.
The Tile mode is great for experimenting and creating cool image effects. You can customize the Tile zoom, Offset X and Offset Y options to achieve the desired results. The tile zoom enlarges the picture, Offset X is changing picture position horizontally while the Offset Y is changing the picture position vertically.
The last scaling option and our newest addition to the aspect field is the Crop feature for which you can find a more in-depth tutorial, here.