Select the gallery under “manage gallery”, then in the drop down menu called “bulk actions” one is “set watermark”. tick the box under that to select all thumbnails or select individual ones, then choose “set watermark” under bulk actions and hit apply.
One annoying thing I found however….when you configure the watermark in the “other options” section, the thumbnail preview image isn’t exactly what it looks like (but its close) and after setting the watermark I wanted to tweak the position and font size but everything you do that, it puts a new watermark on top of the old one so your picture can look like it has multiple watermarks….ugly. Best way around that (but its more work) is to set the watermark to just one image in the gallery and preview that…in order to tweak it I had to use the bulk action called “restore from backup” to give me a clean image, then apply the new watermark to it. Once you’re happy with it, then select all the other images but NOT the one you just did, and apply the watermark to those.