I have an image upload field linked to a core field. I set it to either thumbnail or thumbnail + link. I try uploading my image from my DynReg profile page. Looking at the upload folder via FTP, instead of an image and a thumbnail, I see DynReg has put two differently-named but identically sized versions of the full-size photo. Neither is denoted by a tn_ or thumb_ prefix. The DynReg module shows a broken image.

DNN 4.9.3, DynReg 3.20.60