Paint should work- at least the version with windows 7 will allow you to size to exact pixel dimensions and save as .GIF.. If you're already saving as .GIF and already at 200x200, the problem may just be that the file size is too big, so either reduce the resolution (to 150x150, for example) or if you use GIMP you can reduce the number of colors.
Depending on the image it might also work as a JPG.. experiment with it. I had to try a couple things with my stupid icon.
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