gui design - What's the least confusing UI for offering to either enter a URL or upload a file?

I have a simple form where a user may contribute a link or upload a document adding a title and optionally some notes.

What's the best UI for allowing for either a URL or a file (but not both)?

Answer

Since this is a form, I would actually just provide two buttons inline to choose the image, and not a separate dialog or a set of radio buttons.

I believe this is actually better (in this instance) than having a single image button which opens a dialog to choose the option to then choose your picture. If you're choosing from your computer, the button on the main form would allow you to instantly open the file browse dialog as opposed to forcing the user to go through another dialog. Also, unlike radio-selection options (like the StackExchange option) both options are clearly present up front and are given equal weight.

An example of this would be the imgur upload control, shown below. I might update it to make it more clear that the "Computer" and "Web" buttons are a picture source by indicating "From Computer" and "From Web" or something similar.