Gallery has the ability watermark images or entire albums of images.  This
  is done by overlaying an image on top of another image.
 
  Limitations:
  
- 
     NetPBM does not support steganography like ImageMagick.  Therefore,
     only visible image watermarking is supported.
   
- Warning: Gallery does not store the original image.
     When an image is watermarked, it is permanent, and can not be un-done.
     It is assumed that the user will archive any images that they want
     without a watermark.
   
- 
     Only .gif, .tif, and .png are supported watermark file types.  These
     formats support either a transparent color (gif) or an alpha channel
     (tif and png).
   
- 
     The size of the watermark, in pixels, is the same for the full size
     and resized images.  In proportion to the entire image, the full
     size image's watermark is smaller.  This could
     be a problem when printing the images.