I made most of my 90x120 thumbs with a simple javascript+php app that lets me drag a box around and resize it... all browser based. Submit the crop, enter the gallery URL and then it's added to the pool.
Lately I have been doing thumbs manually with Photoshop. There's a lot more control over brightness, contrast, colour-cast correction, sharpness, quality of save, etc. I've found that most images need a brightness push at the very minimum (many single girl sponsors put out dark images). I've set up a Photoshop action so the process from loading a full sized image to saving a finished thumb doesn't really take much longer than the basic crop via the web interface. Probably 10-20 seconds for most, a little longer for images that need more correction.
Here's an example of before and after correction:
If you don't feel like doing the thumbs yourself then there are companies you can outsource this to.