Put your document in layout mode so you can more readily see the margins. Drop your image in your document from the media browser. Scale the image and move it to the literal edge of the Pages document as shown in the accompanying images:
Notice the document rides under the pale outer edge of the document as indicated by the orange box. When you save this as a PDF, it will look the same in Acrobat Reader XI or Preview. The black border is literally the edge of the PDF document.
Your advice did not at first seem to help at all; I have never been able to produce the orange box, so that I wonder if we are referring to the same version of Pages. (I am running 09.) However, the happy ending is that your comment kept me "in the game" until suddenly things started to work, and I am getting the desired results. After that, I had to learn to put everything in as inline images so they don't jump toward the top of the file when I add space above them. Pages is "deep."
As I can not see the screensnaps I can not be 100% sure of what you are referring to, but unless something has radically changed in the very latest Pages '09 v4.3, Pages does not have bleeds.
The way around this limitation was to make the actual page size large enough to include both the bleed and crop marks to indicate the true trim. Much as we used to have to do in the 1980s.
Peter, I am reposting the images here in response with hope that you can see them, in context with my previous description to Thomas.
Here, the first image is shoved all the way to the right in a Pages v4.3 document. I used an orange annotation rectangle to show where the image submarined under the document edge. The vertical black line is the margin boundary from layout view.
The last image is the exported document viewed as a PDF, where the image remains flush against the document edge and black viewer background, representing a faux bleed.
The grey area is only the non-printable area for the printer and even though the image may extend beyond the page boundary (the black line), that will not be printed from any pdf created, nor directly as far as I know.
Swift Publisher 3 has bleeds, crops, imposition and more. Pages does not to my knowledge.