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[ZiNgA BuRgA]

You can use a text box field instead of a file field if you strictly must pull images from an external source.  However, thumbnails won't work, because, as explained above, it's impossible to do anything with an image unless you have a copy of it.

The file field has the ability to pull stuff from URLs anyway, which does allow thumbnail generation, so I really don't see the problem here.