A couple of days ago, I posted about a website which allowed you to bulk resize images; that site has a sister site which allows images to be watermarked in bulk.
The Batch Watermark; you are able to use an image or text as the watermark and it will be applied to all of the uploaded photos:
I am not a huge fan of watermarking images, but lots of people will use any photo posted online as if it was copyright free, so when I upload photos to my Coppr|Distilleries to Visit and Dark Forge | Tourist Attractions sites, I will apply a watermark to them so any other use of the photo will at least include the original source.