If your images are in JPG form, I'd recommend Easy Thumbnail creator.
http://www.fookes.com/ezthumbs/?Easy%20Thumbnails=2800It's the best one I've found, the only downside is they don't convert gif files. I even emailed them to see if they'd be changing that, and they said they have too much on their plate to do it anytime soon.
It's freeware, and you can even convert all your files at once. I also did something cool with it; you can change the file sizes in the Easy Thumbnail window to see which size will fit best on your pages, before converting the files. So try a size out, then drag the Easy Thumbnail window around over your webpage to first see if the size will work, then when you upload the picture to your webpage it will look the same. You don't want to convert all the images, then have to delete them and redo them if they're too small. It kinda takes most of the guess work out.
I used this because to tell you the truth, I don't think the MC thumbnail converter is all that good. Different images come out in different sizes, and you have to guess what sizes you want. Then it takes awhile to upload (depending on how many images you have uploaded: I have over 600 and it's snail-slow). Then you have to get out of admin panel to your storeview mode to see how it looks. If it doesn't you have to redo the process.
However, I still had to use it for gif images. I just put more effort to getting my product pages in JPG form, so not too many were gifs.
It's a huge time saver. Hope that helps.