Make sure to get the Digital File Pro include from the powerstore. It includes a function that is not included in the latest version of digital file pro, so it may have been overwritten when you were editing the Digital File Pro functionality on the page.
If you are editing with the most recent version of Digital file pro, you can set the "Fit to box" settings to 400 x 400 and remove the resize line added in powerstore.
Powerstore was originally created with DFP v1 which didnt' have resize options, so we added a quick resize function. The more recent version has all that built into the extension and uses different function names.