Hello humphshattered,
welcome to the Pages Discussions. It depends on the output of your document. Do you want to print it by yourself or by a professional printer or is it for screen only?
If you want to print it by yourself, you have to set the page size 8 to 10 mm (ca. 0.39") smaller than the normal page appearance. This is because the most printers print areas are 4 to 5 mm less on every border than the paper sizes are. If you print on DIN A4, you have to set the header, the footer and the text area 4 to 5 mm smaller on every border. Now you can place the image over or at the page borders.
If you want to print it by a professional printer, you have to set the page size 2 to 3 mm (ca. 0.11") wider than the normal page appearance. That are the normal distances. But the best is to ask the publisher/printer about the size.
If you want to publish your document for screen only, you don't have to change the page size, nor the header or text area. Just make the image a floating one (infopalette/wrap inspector/"Fixed on page") and position it at the corner or at the edges you want to have them. Objects with the setting "Move with text" can't be placed at the edges, because you can't make the header and footer disappearing. You can make them as thin as possible by inserting a 0.1 point sized font and set them to 0.0 distances to the page borders, but they are there.
And you can set a mask for images to show only a part of it.
I hope that answers your question.
Frank.