If you want to add images or other attachments to the columns on the master page, you must make sure to reference page they are attached to.
Please note: Due to a link editor feature in Confluence, you cannot display or link to attachments on the .zen.master page itself. Move the attachment to another page and link to it there.
For example, if you refer to attachments on another page, reference the page title in the link editor's Advanced tab, like so:
My Other Page^cute-puppies.gif
My Other Page^white-paper.pdf