View the sword in GM Viewer. Switch the grid off. Rotate the model and zoom until you have a decent view of it. Then press "Print Scrn" to take a screenshot.
Paste that into Photoshop or GIMP. Then you can resize, rotate and copy it onto the interface picture. You'll need to cut away the grey background, then brighten up the sword a bit - GM Viewer's picture is rather dull, even for the ebony sabre.
For the shadow, create a new layer between the interface background and the sword, then use a soft brush to draw the shadow in black. By having this layer under the sword, you don't need to be precise and avoid the sword.