Frank,
If you prefer running in the shell with using the 'wolfram' command, then you can launch the image viewer also from within the WL like so:
p = Plot[ Sin[x], {x,0,1} ];
Export["test.png",p];
Run["gpicview test.png &"];
Or if you want to open it in Midori (default browser on Raspbian), then you can change the Run command to use that:
Run["midori test.png &"]