On the right hand side under your channel box you have a create new layer option create a new layer then select your planes and add them to the layer. Once you have them added you will have 3 squares have the middle one set to R for referenced.
I think the free dragon image plane tut shows you all the steps if you cant get it from my words.
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb