Hello,  I am doing a square 2 tiered cake, the bottom will probably be 8 inches. I want to put 4 buttercream transfers around the sides if the bottom tier, one on each side. I would like the images to be 4 by 3 or 4 inches. My question is would it be better to cover the cake in fondant or buttercream? I am concerned that the images will slide down the sides of the cake ( I live in Arizona & it is mid July lol )

Any advise on how to get bc transfers to stick to the sides of a cake would be much appreciated! Also how do I upload my cake pics? Off topic I know but I can't figure it out!

Oh gosh!  I just now see your post.  Ok, I've done this before but I used buttercream on buttercream or fondant on fondant.  If you cover with buttercream I would suggest that you stick your transfers on the sides after you final frost your cake (on fresh frosting) and then stick that cake in the fridge to let it set.  I've never tried bc transfers on fondant but you could experiment with a little fondant with a slight dab of water for sticking.  Good luck on your cake! =)

Did you pipe a border around the transfer to help it stick?  

There was no need for that.  But make sure that your transfers are frozen before placing them on the sides of your cake.  They need to lay flat on the surface because once the buttercream thaws it would be just like any other decoration on the cake.  It's up to you if you want to pipe a border on or around it but make sure it sticks well.  Good luck! =)