It's nice to share. When you were a kid, mom told you to share your toys. Helped things run more smoothly in the sandbox. As an adult, sharing is still a pretty good idea. Letting your lunch companion nibble on a couple of your fries is a good thing. So how far do you want the sharing thing to go? If you share your french fries, you have pretty good control over what happens to them once they leave your plate. Share your vacation photos on the Web and you don't have that type of control.
PhotoWham is a tool that lets you watermark your pictures. It's nothing fancy: just the ability to label those photos, drawings, or what-have-you so that folks know whence they came. Just enter your text—including a copyright symbol if you want, and press the magic button. You can do individual images, drag-and-drop a handful at a time, or browse to a directory and grab 'em all. In addition to branding those photos, you can do some image resizing and even rename files or move them into a new folder.
Available for free for home use, you should be able to use PhotoWham with Windows XP and later.
Download PhotoWham