Better then crop! In PhotoMix you can increase and decrease your photos within frame. It gives you better results then crop because you never actually lose any part of your photo. It's just hidden.

Select a frame, position your mouse cursor over it, press left mouse button.
In the top left corner of the selected frame, press magnifier button - the image will increase by 10%.
Press and hold left mouse button on magnifier button - the image will increase automatically.
Once you have increased your image, two more buttons appear next to magnifier button.
The leftmost button restores the original size of the image, while the button next to it reduces image size within the frame.