I've got a form which has an image based key. The idea is for the user to choose an image from the drop down menu so that image is displayed in the dropdown field once selected.
This is not a simple task. The built-in drop-down field does not support images.
Doing it with a script is possible, but complicated. You will need to use button fields to hold the images, and then another button to display them, and set one to save the selected value somewhere (in a hidden text field, for example), and then hide itself and the rest.
I don't think there's a tutorial available about this anywhere, as it's a very niche feature.
If this is something you really need to have your best option is probably to hire a professional to create it for you.