Putting a watermark on image using Imagecache Actions
You can use Imagecache Actions module.


  Effects provided
  
  
  Watermarking - a much requested addition that enables you to place any image anywhere over a source picture, with alpha
  transparency
  Overlays - Add photo-corners etc to the image
  Text overlay - add a short copyright notice to your image, using available system fonts
  Color-shifting - Colorize images. Also invert.
  Brighten/Darken
  Alpha blending - turn an image into a transparent mask, and back into a flat image against another color
  Canvas manipulation - add padding, either onto color or transparent backgrounds
  Background - Place your image onto another source background
  File Format switcher - If you need tranparency in JPGs : make them PNG. If your PNG thumbnails are 30K each, save them as JPGs. (This is
  distributed as part of the color-actions submodule)
  Rounded corners (transparent). With input from canaryMason, we can even set different radius for different corners, and round just one
  corner etc.
  Aspect Switcher new 2009-08 - lets you perform different actions or use different dimensions based on whether the image is 'landscape' or
  'portrait'
  Custom Actions - Execute your own PHP/GD code in the middle of an imagecache generation process. Requires some familiarity with imageAPI
  internals.
  


I have bolded the ones that would interest you.