right now the Thumbnail.java class resize images with quality 100 which is not very good because files are very large. the default quality should be 85
and user configurable from the configuration files.

code is this and cannot be changed without creating a patch plugin.
int quality = Integer.parseInt("100");
quality = Math.max(0, Math.min(quality, 100));
param.setQuality((float)quality / 100.0f, false);

also the resizeimage method uses g.setRenderingHint(RenderingHints.KEY_ANTIALIASING,RenderingHints.VALUE_ANTIALIAS_OFF) which means that the created thumbnails look very "pixelated".