If a container field in your "products" table contains the picture of that product, you can add a portal to all products and set up a button in that portal row that uses the Product ID of the row that is clicked to create a new record in the line items table and thus it appears in the portal to items selected. You can also format an object in the selection portal that looks and functions like a check box and then you only need one portal if that sounds like a good idea to you.
To get a portal to all products, use the X operator instead of = to link to a new Tutorial: What are Table Occurrences? of Products.
See the Checkboxes layout of this demo file for an example of what I am Describing: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
If you are using FileMaker 12, open this file from the File menu to get a copy converted to the .FMP12 format so that you can open it up and examine it.
Thank you very much for the quick reply... I am kinda new to this so I'm still tinkering. I will try what you suggested and update this thread.
It may take a while since I'm not that familiar yet, but really, I appreciate it.