if you don't want to buy any software, just use what came on your Mac. Preview will do what you want, just not as fancy as some apps. Just drag all the photos you want to watermark to the dock and drop on Preview, they should all show up in the side bar now. Drag the small image you want to use as the watermark and drop on Preview in the dock next. Select the watermark image and Copy to the clipboard, now select a photo and use the rectangle selection tool to draw a box on the photo about where you want the watermark and Paste in the small image , resize and move if you want to before clicking off the watermark. This is not the best way but will work till you can get a real watermark App.