Views... no format options in Fields->User?
The short answer is that the user image is not a field unlike images attached to nodes. Views is using the field formatter that applies to fields and the way they are themed to allow you to select how you want to theme the field. This functionality doesn't work for properties like the user image.

The same goes for the say the node type. You can't select how to display the node type.

The easiest thing to do, is properly get the raw url and with theming turn it into the image with the image cache preset you want to use.