A portal indicates that you have a relationship between two table occurrences located in Manage | Database | Relationships.
What is that relationship?
I would guess that you have something like this, but with different names. I'm naming things by where they are found in your solution:
LayoutTO::__pkLayoutTOID = PortalTO::_fkLayoutTOID
Let's add one more table to this like this:
PortalTO::_fkPicturesTOID = PicturesTO::__pkPicturesTOID
Put your container field in Pictures, not one of the other two tables. Define a PictureName field and enter the name of the picture, use the Unique Values validation field option to ensure that each picture has a unique name.
See the first post of this thread if this notation is not familiar: Common Forum Relationship and Field Notations Explained
You can then place _fkPictureTOID in the portal row formatted as a drop down list or pop up menu. Give it a value list with __pkPicturesTOID as the first field and PictureName as the second field. This allows your user to select a picture by name. Put the container field from PicturesTO in your portal row and when you select a picture name in the drop down, the selected picture will appear in this field.
Thank you for your help with my question. I am now able to view pictures in the container field. Your reference to notation was also extremely helpful because by using it throughout my database I have not only simplified some table fields but also found errors that were creating problems.