This cannot be done with FileMaker. It's one file to a container field. But you can set up a related table with a container field such that you can create as many related records as you need and put one image file into the container field of each record.
few questions here:
When you say 'related records' then I have to create a new record (record B) and then link it to the other one (A) to show the photos from B in A?
I don't know how to create relationships at all; been working on it for days and gave up and just switched stuff in the layout from text to numbers, containers to text, etc. Don't know how to do anything else but work with what I'm seeing.
You would create a new record in a different related table.
You'll need to learn how to create tables and link them in relationships if you expect to use FileMaker to do what you need here.
Open Manage | Database | Tables
Enter the name of a new table here and click create. Click on the fields tab and enter the names and details for each field that you need. For a table of images, you'll need a minimum of two fields: An ID field to link to your inventory table in relationships and a container field for inserting images. If you do not have an ID field in your inventory table that uniquely identifies each inventory record, you'll need to add one. This field is called a Primary Key. Define it as a number field with an auto-entered serial number. (Double click this field in Manage | Database | fields and select this option in the Field Options dialog box.) (Note: if you have existing records in the inventory table, adding this new field will not also assign serial number fields to them. Replace Field contents from the Records menu can be used to assign a serial number to each record that already exists in your table.)
Click over to Relationships where you'll find a Table Occurrence "box" for each table that you've created in your file. Drag from the ID field in your Inventory table to the ID field in your new images table. You've now linked them in a relationship. Double click that line you just created linking these two table occurrences and select "allow creation of records via this relationship" for the Images side of the relationship. This is not strictly necessary, but it makes life simpler for new database developers.
Now you can add a portal to Images on your Inventory Layout and insert images in to the container field from Images that you can place inside the portal row. I recommend that you read up on "portals" in FileMaker help and any training materials you have available as this is a very frequently used layout design for working with multiple related records.