Pages picture frames printing at dismal resolution. Help!

I was about to go to print with a theatre programme I created (perhaps ill-advisedly) in Pages 5.5.3.

I have used the built-in picture frame options around several pictures. Specifically the one with metal clips, and the one that looks like the picture is flexing out of the page. (Don't judge me on my design choices 🙂 )

On-screen everything looks fine. But when I print, the photos are in high resolution, but the picture frame elements are rendering in very low resolution - I'd guess about 25-50dpi. So the pictures lose their crisp edges and the graphical frame elements look like they come from the 1980s.

The same happens when I save as PDF. The attached image is a screen grab from Preview at about 200% zoom. I've tried several different picture frames and they all exhibit the same behaviour.

Note that all is fine on screen in Pages at any zoom - they render perfectly. It's only when outputting that it all goes to pot.

Please help! I need to send this programme to print and at the moment it's going to look awful.