To open this window, select Create > Stencil from the main menu bar. See also Create a stencil.
Only available when you have an image copied to your system clipboard. When on, Mudbox pastes the image you have copied to the clipboard (from another program such as a web browser) onto the stencil.
When pasting an image from your system clipboard, the Width and Height fields are disabled and the new stencil size is determined by the dimensions of the copied image.