It is a pretty flat database... On each record I have 6 container fields,
That's Problem #1. You should consider setting up a related table where each record has a single container field for a single image. That offers a great deal more flexibility.
I don't know if this is truly the case, but it sounds like you want something fairly similar to an invoicing system where you can select multiple items to be sold on the invoice and you want to see a picture of each item selected for that invoice. You might be doing something much different than buying or selling here, but in terms of the interface--that's my best guess on what you are doing. It might be a purchase order, a Bill of Materials (BOM) or something else, but that select a list of items and see their picture part of the process is quite similar in each case.
MainTable::__pkMainTableID = SelectedItems::_fkMainTableID
Items::__pkItemID = SelectedITems::_fkItemID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
With such an approach, you'd define a container field in Items into which you would insert your pictures. Then you'd set up a portal to SelectedItems on your MainTable layout and include that container field from Items inside the portal row. When you select an item in the _fkITemID fieild, the image for that selected item will appear in the container field in the portal.
Please note that in some situations, such as the BOM that I mentioned, MainTable and Items can be two Tutorial: What are Table Occurrences? of the same data source table.
You can examine the invoices starter solution for a working example of this setup and how you might select items in the portal. (It won't show pictures though). And I'm not ignoring the fact that you've mentioned that you have approximately 1000 pictures here either. The drop down list method in the starter solution can be less than ideal for long lists of values from which to choose, but once you get those basics in place and working, you can explore various other options that work with the same basic design but make selecting an item much more user friendly.