Hey there Peter,

I'm assuming that you've attached an image plane to a camera and you're trying to delete it. If you select the image plane and press delete Maya will think that you're trying to delete the camera which, unless it is a camera that you've created, you can't.

What you need to do is select the camera and scroll down in the channel box until you see the image plane inpute node, select that and delete it, job done.

Take it easy,

Mat.