External Storage has a lot of good features so I wouldn't automatically rule out that option.
One way to improve performance is to stick with lower res thumbnails of your images wherever possible on your layouts. You might, for example have a "gallery" of multiple images displayed all at the same time on your screen. Sticking with lower res thumbnails on this screen will make it update much faster than the full higher res images.
Once the user selects a single image to view, you can opt for the original hi res display of this image.
So if I use the default options with external storage... Filemaker will be able to host my files on my fileserver? Will these files be encrypted or placed in some sorta database file? I'd prefer them to be raw. So I have the option to access them directly too over the network.
I remember in the past I've attempted uploading a larger file into a container field and filemaker sorta just hangs there while 'uploading'. Are there any further ways to make the uploading / downloading process more user friendly? ie. progress bar or something.