You cannot crop an image from the general deck editor section... yet.
There are 2 places to crop images:
1. When you are setting up your avatar, you can crop the image right after you pick your image.
2. in the Image Manager section...
- select the image you wish to crop,
then tap the "Edit Image" toolbar button.
Note, when editing images in the Image Manager, you are changing the file permanently. If you need 2 copies of the same image with different crop settings... you will need to upload the image twice and crop it twice.