The closest thing to masking is probably to split your image into multiple texture controls.
You can let each control display a subportion of the image via SetVertexUV, so you can create a grid of controls to display the entire image.
Then you can set the alpha of some corners of some controls to 0 via SetVertexColors (or hide an entire control via SetHidden) to create a poor man's version of a mask.
That's how i did the "masking" in TrueExploration: http://www.esoui.com/downloads/info5...ploration.html
Here I display a parchment image above the map and mask the areas that were already visited by the player to create some kind of Fog-of-War effect.