Masking an image works like the alpha channel in photoshop: use an image in 8-bit (black to white), the white will still in the output image, the black will be transparent, and all level of grays will be translucent colors.

<mask path="path to the image" [stretch="none/horizontal/vertical/both"] [position="placement"] [pattern="true/false"] [margin-top="size"] [margin-bottom="size"] [margin-left="size"] [margin-right="size"]/>

The possible parameters are :

• path : a relative path to the image you want to put on top of the original image.
  Ex: /Public/images/logo.gif
• stretch : indicates whether you want the new layer stretched and in which direction
• position : indicates the position of the new layer.
  Possible values are :
• topright
• topleft
• topcenter
• center
• centerleft
• centerright
• bottomcenter
• bottomright
• bottomleft
• pattern : indicates if you want the new layer tiled all over the original image

Ex: a blue crosse tiled all over my original image
<mask path="/Public/images/blue_cross.png" pattern="true"/>