Hi there,
i know this question is a bit old but maybe someone will find it helpfull: If you want to clamp an image to a spheremask you would use a skyboximage-node in material editor. You can place an image at your sky-material using a 3 dimensional vector (similar to spheremask + cameravector and dotproduct) for positioning. Furthermore you can adjust the scale and rotation of your image. The node gives you the rgb (tiled and repeated across the whole geometry) and apropriate masks to remove the tilings.
You may checkout this answer: Updating Custom Sun Texture Using SkyboxImage - Community & Industry Discussion - Epic Developer Community Forums
regards!