I am doing a Bday for a little girl who wants Princess brownies but hates any type of icing. Will edible images work in this case?

I don't see how they could work. First, placing an EI on a dark background can make the picture almost impossible to see, unless you're using those ridiculously thick EI sheets. Even if you can see the image, the brown background might make some of the colors look weird. White background is the best way to go.

Second, brownies are bumpy. So, your image will also be bumpy.

I would suggest mounting the images onto thin pieces of fondant and allowed to dry. They can be laid on the brownies or mounted on a stick to stand upright.