Marekkis Watermark plugin for WordPress
Marekkis Watermark-Plugin for WordPress can watermark your pictures an two different ways:
After the activation every picture that you will upload with wordpress build-in media-uploader will be watermarked.
In the configuration-screen you can set up the position and the type of your watermark. It can be your logo (.png-file) with transparent background or a free text with color, font, size, shadow and transparency-level of your choice. See screenshots.
Marekkis Watermark makes possible to create a watermark on mediafiles that are already uploaded on your server. So you can mark all your old pictures with the new watermark.