When exposing original-size or otherwise huge file versions *for download* when they're not intended to be viewed in browser, it may be wise to use the 'Content-Disposition' HTTP header, as we do for XML output from Special:Export.
This should force most browsers to show a download dialog instead of loading the giant file inline; you can then take your mega-image and stick it in Photoshop or Gimp or whatever if you really wanted it (such programs are optimized to work with huge files, while browsers are most emphatically *not*).
Of course this may mean we need to either filter the file through PHP to add headers (scary!) or have a second "downloadable" URL where the swift or caching infrastructure adds the headers.
-- brion