I know I've done this before but it's been a long time. What is the best way to stick an edible image to fondant??? I'm putting it on the side of the cake.

I use piping gel

what about applying to BC. i have an edible image to put on BC. not sure if i should put it directly onto the BC or use something inbetween.
TIA

On fondant I either apply it right after I roll it out (before it crust) or sprits some water lightly on it and then apply the image.

As for buttercream, you can apply right to the buttercream, again it is easier when it has not crusted yet, but if it has crusted to much then I sprits with water.

Be careful with spritzing with water though on either medium, if you get to much water then you can get water spots on your image. If you get to much water on there just wait for to dry a little bit before placing the edible image.