Unfortunately, you can't do exactly what you describe in FileMaker.
Drop downs in FileMaker can only store text--not images.
You can make a drop down of names for these images with a script that adds the image when the user selects it. Or you can load the images into container fields--each in a separate record and display them in a portal. The field can be set up as a button to perform a script that does what ever steps are needed to select that image for the current record. It might be as simple as this single script step:
Set Field [MainTable::containerField ; PortalTable::ContainerField]
For this type of use, you may need to use store by reference container fields to keep your file size manageable.
or better yet:
Set Field [MainTable::ImageID ; PortalTable::ImageID] //requires a relationship that uses ImageID to link the two tables
Thanks for your answer and sorry for the delay somehow (as often is the case) I did not get an email that you posted.
I need to give this a go as I am not sure I fully understand your instructions. I will post back.
I Managed, thank you!