PhotoWatermark Professional is the only product on the market that can watermark animated GIF frame by frame and save them back as animated GIF. To enable this support, you need to
1) Turn Options/Multipage image support
2) When you save your pictures, you have to specify either GIF or As is as the conversion tab.
If you choose to save them as JPEG, Bitmap, PNG or JPEG 2000, each frame will be saved as a separate file.
See also:
Features