I think DefIcons comes pre-configured for most common filetypes, such as JPEG. Check in the prefs of DefIcons, look for JPEG and see what action it is set to carry out... Change that action to use whatever viewer you like on your system and it should work. Of course, JPEG files *with* an icon/.info file will use the default tool set in the .info file instead. This allows you to override the DefIcons settings for specific files.
I personally think the Amiga system is great once you get your head around it. And RaWBInfo makes handling them a lot easier, allowing you to drag 'n' drop tooltypes etc. in Workbench.