Imagemagick is an utility used to create to create thumbs, it have maybe parameters and options, you can take a look at full doc at imagemagick.org, so you can edit Crop Profiles in script and set your own parameters for IM to get better thumbs.

Some examples:

-filter Blackman -modulate 105,102,100 -sharpen 1x1 -enhance -despeckle -strip -quality 85
-filter Sinc -modulate 105,110,100 -unsharp 1.0x1.0+0.1 -enhance -strip -quality 85
-filter Blackman -modulate 110,102,100 -sharpen 1x1 -enhance -quality 85