The deficons program reads its settings from the deficons.prefs file which is stored in envarc:. The deficons preferences editor does nothing else but to edit the deficons.prefs file. As already said the preferences editor only exists for OS 3.5 and above. But the default deficons.prefs file which comes with the newicons package already contains most needed file types.
Therefore all you need to do is to copy deficons to c, deficons.prefs to envarc and add a line to s:user-startup which runs deficons. Then you can create an icon file called def_jpg.info with your desired default tool. The icon file needs to be stored in envarc:sys so that deficons can find it.
Now whenever you select show all files from the Workbench menu, all jpg files should be displayed with your def_jpg.info icon with your default tool. And I am sure that deficons does not rely on the .jpg extension, but also shows the jpeg icons for files which are not called .jpg.
