If I make all thumbnails the same size, it will be difficult to distinguish giant planets of terrestrial and selenas. Thumbnails are already drawn not-to-scale: radii of planets is logarithmed...
Well, how about keeping the current thumbnail ratios the same, but increase the whole system browser in size by 50%, so everything looks 50% bigger? Then at least the smaller planets/moons would be seen in much better detail. I understand that this browser size increase would not be a problem on smaller monitors because the images can be scrolled left and right if they should go off-screen anyway, if I recall this correctly.
Barycenters - yes, they should have an icon. I'm not sure if that icon should be a simply image file, or generated in engine like the planets thumbnails (maybe both stars fitted into thumbnail with orbits rendering enabled?)
If those images are generated in engine, the barycenter would still be a problem to display anyway; it's still an empty thing, even if it had stars/planets generated with it too. I would suggest using just a symbol for it, such as a 2D yellow marker cross with 2D celestial bodies around it... maybe?