Secure Random Adoption V2

[Kesstryl]

I'm not sure if this is the place to ask, and if not, let me know, but I would like to add an extra feature to this code which makes some of the random pets more rare than others. I found this code on this site which uses a 'chance' variable which will decrease the odds of the random pick on some of the images:

http://www.chickensmoothie.com/tutorial/tut3/index.html

If anyone has time to take a peek and let me know how this can be added into this mod code, I would be very grateful as I'm good at following directions, but not so good at knowing how to do this myself.