Open the picture you want to the scene preview out of
Select an area that is 144 by 81
Copy the selection (edit>copy, or ctrl-c)
Close the image, and open a new image.
Make sure width is set to 144, height is 81, and mode is RGB colour.
Paste in the image.(edit>paste, or ctrl V)
Flatten the image (layer> flatten image)
Now, goto Image>Mode>indexed colour.
Under palette, select custom, the click on load.
Select the Photoshop colour table in your 3dmm picture/palette directory.
Click ok.
Now, select Dither>Diffusion,and set colour matching to best. Then click OK.
Now, save the image as a BMP. (File-> save a copy)
Set File format to windows, depth to 8bit, and make sure compress(rle) is off
There, now go back to the main Doc.