The next time you save an image, Reddit won't add its watermark. This will also allow you to save the images in better quality, as the watermark appears to degrade the image a bit. (Another excellent reason to disable the feature.) You should know, however, that when the Reddit app updates, this setting is sometimes reset, so you may have to disable it again periodically. In case that bothers you, you can always take a screenshot of the image on your iPhone or Android phone. That way, you'll always circumvent Reddit's annoying watermark.