What is the best way to attach an edible image on fondant? Thank you.

I've only done very small images but I brushed on a tiny bit of water just to moisten it. You could also use vodka or an extract if you are worried about it getting too wet.

I've only done it on one cake, but I brushd it lightly with piping gel.

the use of something like vodka is best, but either the water or piping gel will work. Water is better than piping gel.