Brief tests using Insert > Choose and Copy and Copy>Paste suggest that choosing an image from the Photos folder is the only path to inserting an image file into the photo container in a Pages template.
That in turn suggests three possible alternate means of getting an image into a Pages document. Both involve deleting the photo container, meaning you will lose the ability to mask the photo.
Delete the placeholder photo and its container.
Use the Insert menu > Choose item to open a finder window in which you can locate and choose an image file to insert in the document, then resize the image and place it where you want it.
OR
Open a finder window, locate and Copy an image file. Return to the Pages document and Paste the file to the document, then resize and relocate as above.
OR
Locate the images you are expecting to use in the document, and place a copy of these images into the Photos folder. Use the built in path to move that image from the Photos folder to the container, replacing the placeholder image in that container. Resize or mask as needed. Change the location if the container is not placed where you need it.
Those appear to be the options. Others may be available (As mentioned, my tsts were brief), and may yet be added in posts from other users..
Regards,
Barry
PS: Thanks for noting your macOS version number.
Please add the version number of Pages you are using (Pages menu > About Pages), as this may be significant in determining a solution.
B.