Lots of times I've wanted to put on my picpix/pics.lj account a picture that's already on some web server somewhere so that I can guarantee the URL won't change, that they won't block “remote loading” and so forth. However, right now the only way to do that is to save it to my hard drive and upload it from there.
It would be nice to be able to specify a URL and have the server go and fetch the image, much like LiveJournal's userpic fetching. The main issue to contend with here is that it may take several minutes to retrieve a full image so it would have to be an asynchronous operation which puts the image into a specified gallery when the download finishes. However, the transfer starting must be a synchronous operation so that messages about things like unsupported formats and HTTP errors can be reported to the user. I'm imagining some kind of scheme where the server makes a request and gets back the HTTP header but then hands off to the async process to actually complete the download if it's acceptable. I think this is something that Perlbal could help with, perhaps.
However, this got me thinking that it would be nice to be able to import pictures en masse from a gallery hosted elsewhere. The easiest way to do this, I think, is to either find or invent some XML/whatever format for describing photo galleries and support the consumption of images listed in this format. If the format specifies both thumbnail and full image URLs then an interface to do this could show all of the images and allow the user to choose which ones to import. Fotobilder can then support publishing such data so that Fotobilder installations can import images from each other. I would also probably be inclined to extend Gallery to export data in the sharing format.
My first single-image idea could actually be implemented client-side as a browser extension, since by the time I'm looking at the image I've already downloaded it anyway. Importing multiple images without having to download the full images first would be really neat, though, as I could take photo galleries published by my friends elsewhere and import them into my own without having to download and then upload 100MB of data redundantly.