Looter, you can give one of these a try and see if they work for you. Ultimately you have to find the lowest common denominator that gives you something reasonable since different content providers using different encoding and post-edit techniques. There really isn't a one-size fits all.
In any case, try one of these, which I find does a reasonable job for the most part:
We use this on our Mechbunny TGP Script and it typically does the job, but we have some content providers with small "large" pics and they're already pretty sharp so it's a challenge. This one seemed to let us accommodate a wide assortment of content providers.:
Code:
-modulate 110,102,100 -adaptive-sharpen 0x0.6 -enhance -quality 92
We use this one with Arylia and it works quite nicely:
Code:
-modulate 100,102,100 -sharpen 0.65x0.65 -enhance
We use this one with the older JMBSoft TGP and it works nicely:
Code:
-modulate 101,106,100 -sharpen 1x1 -enhance
Again, there's no one-size fits all. Ultimately, some thumbs will look great and some not so much. And like I mentioned, the key is in finding a balance you can use for multiple content providers.
If all else fails, trial and error is your best teacher. The command line options are here:
ImageMagick: Command-line Processing
Good luck.