UploadWizard used to support uploading from Flickr with some extra Flickr-specific features. It did not, however, allow uploading from other external URLs. MediaUploader has removed this feature completely. Uploading from URL can currently be only achieved using the API or Special:Upload. See also: Manual:$wgAllowCopyUploads.

We could investigate making it a more general feature. I do not have a real use case for this at the moment, but I can imagine some wikis would need that.

Setting this to lowest priority until someone expresses interest in the idea.