Do you choose which two it combines, Or is it random? About the art, you could also ask someone here to make the art instead of using the internet.
1- i think that you can choose
2- do you know someone who can make the art??
1- if you can choose, it would be a little OP since you could combine dragons, to make a huge immaterial dragon thing. Maybe add a poll, asking if it should be random or not.
2- well, there's me, EmeraldTiger, Flayne, vrt, pepokish, and Krava, but I won't guarantee that anyone besides me will help you, since I am not speaking for them, I just think that they are the some of the best artists around here.