There is no direct way to pick an attachment from within the macro, but a workaround is available below.
You can use an image attachment on a page, if you know its URL. You will first need to obtain the URL from the page's Attachments section.
In view mode, load the page's Attachments section by clicking on
Right-click on the attachment image you want to use, then click Copy link address (you will be pasting this URL later).
Click on View page, then Edit the page.
Insert a Page Banner macro. and fill in the various macro fields.
In the Image URL field, paste the URL which you copied earlier. You can click on Preview to view it.
Click Save, then click on Update or Publish to view the final results.
Here's a step-by-step video on how to do this: