Make firesized post thumbnails work with both local and remote images

[curiositry]

There’s no easy way to tell if the image in the {{image}} helper was uploaded by the user into the content folder, or if it’s linking to an external image. This means that, right now, post thumbnails only work with remote images, because firesize needs an absolute URL.

I need to find a way to check if the image is local or remote, and prepend the blog url if it’s local — until the {{image}} helper becomes an actually helpful helper.