In order to convert DDT files to JPEG, you will first need the software which allows you to open DDT files. For this operation, Diagram Designer handles DDT files pretty well, in fact, this is a standard format designed to save template files. Simply open the DDT file in the software and then use File > Export page function and select JPG format from the drop-down menu of the supported file lists.
To assure you can do this, here it is one of the features that the software offers:
Import/export WMF, EMF, BMP, JPEG, PNG, MNG, ICO, GIF and PCX images.