Aha! I just downloaded the seagrass image and see that it is on a transparent background .png -(Rondo did mention that). In order to use it as a stencil you would need to convert to a jpg.
If you don't have any software to do this, i.e. Photoshop, Infraview, etc. you can use AR. To do this - open AR - blank canvas -
File/Import File to Layer then
File Export Image File and in the drop down menu choose .jpg.
You will need to crop the image before saving or use a very small canvas to start with, e.g. 400x500px.