The Marmoleum texture came from Alex Guerrero's WinMan website, which no longer seems to be up.
You can go to any website that supplies textures as bitmap files, though, and download those. (e.g.
http://www.cgtextures.com. Remember anything that is displayed by your browser can be saved by right-clicking, and selecting "Save image as...") The size is not very important, as long as it is larger than the largest square size you want to use. (The largest square size supported by WinBoard is 129 x 129.)
Just download two bitmap files (one for the light squares, one for the dark), and put them in the sub-folder "textures" of the WinBoard folder. (You could put them anywhere actually, as long as you tell WinBoard where they are, but it is better to stay organized!) Say the names of these files are marmo_l.bmp and marmo_d.bmp, for definiteness. Then run WinBoard one time throught the Startup Dialog, ticking Additional loptions and typing in the option field:
/liteBackTextureFile="textures/marmo_l.bmp" /darkBackTextureFile="textures/marmo_d.bmp"That should do it. To switch them off again you should use the same two options, but now set them to empty strings ("") or anything that starts with an asterisk ("*").