Your old photoshop files will still work. If you have any text layers, photoshop will just ask you to update these layers in order for you to use the updated features for text.
Like Rob said, the "Save for web" feature works good to get images to the four empeg colors. just convert your file to grayscale before you use that function.

For the files on rio.org, open them with ImageReady and click on your optimize tab on the file window. Play with the settings on the top right corner of the program. Be sure to have gif and (4) colors selected. The file window will give for a preview of the changes you make. Now, once you are happy with the way it looks, go to File-save optimized as-and your destination.

Upload and test your animation.
Up the contrast if needed

-Loren-
What did you use for fixing the contrast?