It's not possible to have images in a dropdown list in Filemaker. Instead, create a button that takes you to a layout which shows all the images in a list (either a layout in List View or a layout with a portal for the images), then the user clicks one of the images to select it, then it returns to the original layout with the selected image displayed. You just need to create a couple of simple scripts to get it working.
To expand on that, you can also pop up such a list or portal in a floating "Psuedo-modal" window if you want or just place such a portal directly on your first layout if you have room for it.
Thanks everyone for your responses...I will give these a try soon and let you know how it goes!