add photo to document
I cannot add a photo to my document because 'Define as Media Placement' is greyed out
iMac, macOS Sierra (10.12.6)
Presumeably, you are talking about some anonymous version of Pages. You can drag & drop individual images into your document without having any one of them Defined as a Media Placeholder. You could pick, for instance, the first dropped image, scale and position it as needed, set its Text Wrap to None, and then Define it as a Media Placeholder.
Subsequent images that you drag & drop onto that defined Media Placeholder will automatically adapt to its dimensions, and replace the original image. You can do this multiple times. Also, you can have multiple images defined as Media Placeholders.
Your original image could even be PDF artwork.