Try WordPress Aparg Watermark and Resize plugin!
Aparg Watermark and Resize designed for resizing images and placing watermark on them on fly. Images will be automatically resized and watermark will be placed on them when you upload images to WordPress media library. You can choose whether to do both actions or only one of them. You can also perform bulk action. In this case actions will be applied to all existing images in media library. We recommend to make backup of your website before bulk action.