"The User Defined Alias must derive from the first FROM object"?

Content

I have a custom object package named Documents that contains objects named Images and Paragraphs, among others. There's an Images.URL field. The Paragraphs object has an Image_ID foreign key to the Images object. When I query for a Paragraph that has an associated Image, I'd like to be able to get the Image URL without running a second query. This is a basic join, and I could do it in a heartbeat with SQL...but ROQL's a bit different.

When I run the query

SELECT P.Template_Text, I.URL FROM Documents.Paragraphs P LEFT OUTER JOIN Documents.Images I ON P.Image_ID = I.ID WHERE Document_ID = 4