Create watermark with Imagecache_actions.

You can use a module like FileField Insert to insert an ImageCache preset (with a watermark) into the body area, then Image Resize Filter can scale that image down. However, Image Resize Filter cannot (and likely will not) provide the ability to add effects to images after they have been scaled down. If you need inline images with watermarks, I'd suggest you just use ImageCache and FileField Insert and not plan on using Image Resize Filter at all, since you're already using ImageCache to scale down your sizes. But you can use all 3 together, try them out and see if that will fit your needs.