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.