It was on an issue of the Adventurer - the LucasArts catalogue that shipped in US copies of the games in the early 90s. See
here 
As I understand it, Laserschwert has reconstructed these images from hi-res scans of boxes and magazine ads, and some other sources in certain cases. I believe there were only official released posters for the first LucasArts games - Maniac Mansion, Zak McKracken, Indiana Jones and the Last Crusade (?), Loom (?) and the Secret of Monkey Island. Those posters are
extremely rare! I've never even seen one for sale. Well, except the Maniac Mansion ones - but a few years ago, the LucasArts Store found a box of those from way back and put them on sale
