There was a discussion about two years ago regarding external image hosting and using nextgen gallery: http://wordpress.org/support/topic/plugin-nextgen-gallery-storing-images-outside-wp-contentgallery
The plugin author wrote: "currently it's not possible to store the galleries outside , because I used the ABSPATH & blogurl as base for the complete path. I planned some rework for a next version, but it will takes some time"
I haven't found any further info on the subject.
Is it now possible to use external image hosting?