Ah I see now. Yeah Poly planes is so much more flexible. Easy to turn on and off and move out of the way. Holds image ratio flawlessly and so on. Only advantages as far as I can tell. A very handy trick is to put the planes on a layer in the layer manager and set these layers to R for referrence. Now you can't click them or accidently move them anymore. Perfect.

The few times I've tried to attach it to the camera approach I learned that Maya does not like .jpg for this. All kinds of weird things happened. Try a .tga and see if you get the same results.

Yeah 3d.sk is cool. However you do typically need to correct the referrence images a bit. 3D.sk does not do this for you. Namely barrel lens distortion that occurs with all photography (fish eye effect), straightening out the axis and possibly correcting yaw-pitch tilt distortions. Here's how:

https://forums.ego-farms.com/index.php?topic=123.0
First and foremost, Grab
Part 01 - Preparing Images for Reference Plates by Eric Maslowski.

The rest are very nice too.