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.