See this post for details on Transform Image, and resizing in particular.
Convert Image supports conversion between various formats, implemented using the ImageIO class, and the JAI library (which adds tiff support).
Stamping an image is not available - you might want to look into using an external tool, and calling it via the command line. One tool which you might be able to stamp an image is imagemagick.Using an external tool, you'll probably want to read/write the file to a temporary folder in your project,. In that regard, feature #284, currently in development, will allow you to retrieve the project's location in the file-system and should prove useful.
For best results, use the Firefox browser..