In wordpress if you need to watermark selected image, ‘Easy Watermark and Image Optimize’ is a good choice.
1.Download ‘Easy Watermark and Images Optimize’ plugin from this link
2.upload this plugin to www.yoursite.com\wp-content\plugins\ folder.
3.Login to wp-admin.In your admin dashboard click plugins and activate the Easy Watermark and Images Optimize plugin.
4.Then go to settings and click imageoptimizer.
5.Type your watermark text and update options.
6.In the image uploading window you will need to specify if you want the image and the thumbnail to be watermarked.