AR reference image resolution low

I'm creating an AR app in ARKit. I've just implemented image targets but in xcode it states my reference images are all too low resolution. When I build and run on iOS (12 if that makes a difference) I get the error "invalid reference image". It also states that all of the images have a physical size of 0 despite me declaring otherwise? All of the images are of a decent resolution, generally around 400 by 400 pixels. When the app launches the camera immediately freezes.