Adding a background image in Pages
If you load the image to the document first, then go to Arrange at the top of the tool bar and choose lock (Command+L), then you can add and edit the text over the image.
If you load the image to the document first, then go to Arrange at the top of the tool bar and choose lock (Command+L), then you can add and edit the text over the image.
The proper means to set a background image in Pages for macOS is:
This positions the image below all body text and objects added to the page in the current section.
See Add watermarks and background objects in Pages on Mac - Apple Support
Adding a background image in Pages