For best results make sure you crop from as large an area as is possible, (so use the images of galleries, rather than the thumbs). Sharpen your images using filtering tools, and compress as little as your bandwidth allows, 60% to 85% on most jpeg compressors.
Comus Thumbs, is a great tool for quickly automating this whole process and making thumbs for TGP's.
It also filters the images to sharpen them up.
And automates all of your TGP chores.
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http://comusthumbs.com - Still the best TGP Script.