How do I crop from the default image viewer?

Once the image is opened you can right click on it and choose to open with Shotwell Image Viewer. It will open up a new window where you can crop/edit the image and save it.
Once saved you can close the 2nd window and see the changes in the default image viewer.

Alternatively you can set Shotwell Image Viewer to be your default image viewer.

The default viewer is very limited, no editing and it doesn't support animated GIFs. What I recommend is installing gthumb which is in the Ubuntu repositories. Gthumb is fast loading and provides all the basic editing functions without the complication of GIMP. It also serves as a image manager which doesn't impose a date based folder structure for images. I for one prefer filing by event or subject -- makes images much easier to locate.