No script should be needed and you can delete the ColateralGraphicsA field from your database. Given the relationships that you have, you can place the container field from GraphicResources on your layout in place of it.
Then format the _fkGraphicResources field with a drop down list or pop up menu of values from the GraphicResources table. Specify _pkGraphicResources as field one for the value list and your Graphic Identifier field from this table as field two. You can set it up to hide the pk field and just show the graphic identifiers in the value list.
This is the "beginner level" way to do this. More sophisticated options for selecting a graphic are possible, but they all are different ways for taking the graphic identifier and entering the PK field from GraphicsReources into your fk field in order to establish the needed link such that the container field from Graphic Resources displays the correct image.
Thanks Phil. One thing that I omitted is that I could have two graphics in a record (double sided - shown in the uploaded image) and they could be different. That is why I had two Container fields (ColateralGraphicsA & ColateralGraphicsB) in the CollateralResources table.
But you don't need those fields for two graphics. You can have two relationships to two Tutorial: What are Table Occurrences? of the GraphicsResources table.
Either that or set up a many to many relationship between the two tables as is also possible. This can be done with either a multi-value key or a join table to serve as the link between the two tables.