I was adding an image over the top of my Layout file to act as a watermark, before I exported a PDF.
As Layout doesn't have opacity control for shapes or text (at least, I don't think it does?), I created an image in photoshop and saved it as 20% opacity. This was too light, so I kept re-saving the file in Photoshop at higher opacities (25%, 30%, 35%) etc until I was happy with how it looked.
Rather than re import the image every time, the quickest way was to overwrite the original watermark.png file, that I'd saved on my desktop, and then right click in Layout and update the reference (similar to the workflow of editing a Sketchup file and updating the Layout document).
I also deleted this watermark at different times, and then copied and pasted it back from other pages within the document.
Edit: Further to this. Leaving the image in the file is fine (and I think it does save). The issue comes when the image is deleted. I also tried Edit>Undo to undelete the broken image. The image comes back, but it still won't save. Also purging the image out didn't bring back the save feature.