Send a copy of the page so i can see the code.
The File Upload server behavior will create a Binding Group named WA Uploaded Files (<File Field Name>)
In theWA Uploaded Files (<File Field Name>) binding group there will be bindings that have the Upload path for the large image at the beginning of the names, and another set of with the path for the thumbnail image at the beginning of the name.
in both sections there is a binding for Server file name.
Use those bindings in the insert behavior.