I figured it out and posted the following in the Digital File Pro forum.
OK, I figured it out after much head scratching. First you use DataAssist to build your insert record page. Then you create the upload file behavior in file manipulation for the form field of your choice. Then you go to server behavior, click add, and select insert from the DataAssist section (not the first insert behavior). From there, you specify your database info and session variable (which is filled in for you). On the next screen, you select the form field you want to use to store the thumbnail image text/file name. Click on the lightening bolt icon and from within the dynamic form part, double click on simple file name. Don't forget to add .jpg or whatever your image extension is as the file upload breaks up the name. For trigger, I selected "any successful upload". That way, once my first thumbnail was uploaded, this behavior would have the file name ready to insert into the record. It works like a charm, but took forever to figure out!
I hope this helps anyone else who might have the same problem.