If the image file you are uploading and the print size you selected have different proportions, the image will be cropped to fit into the print size.
To adjust the crop of an image, first place the image into the working space. You can do this by clicking and dragging an image from the menu on the left into the working area.
Once the image is in the working space, double click on it to open the crop function.
You can then adjust the crop by moving the red area. You can move the crop area by clicking on it and dragging it to the desired direction. You can resize it by clicking on the arrow in the bottom-right corner of the crop area and dragging in the desired direction. Lastly, you can also rotate it by using the slider at the bottom. Please note that the proportions of the crop area area determined by the print size selected: for example, if you've selected a 10x10 canvas, the proportions of the crop area will always be square.