Your cel, and the Rambo Sakuya, Ryoko in scarf 
with the binoculars, Sasami and Ryo-ohki as ship 
captains, the girls drinking sake, Tenchi tied to 
the cross (with just Ryo-ohki or a fan to cover 
his...um, private parts, one was recently on ebay) 
and a number of other 'mystery' Shin Tenchi cels 
are all from a sort of bonus episode that is 
scattered in small chapters throughout the Shin 
Tenchi Lasar Disks, in Japan. They might be on the 
Japanese tapes, I don't know. But a friend was 
nice enough to tape the episode for me, and it is 
hilarious! 
It is totally over the top, and is much funnier to 
me than the Shin series is, it is basically a 
parody of Shin, plus a Bruce Lee movie, I think 
was called "The Tower Of Death". 
 
The story basically is that the girls kidnap 
Tenchi and Sakuya and those two guys go to get him 
back. Sakuya (in Rambo garb) has to fight one of 
the girls on each level of a tower to get to the 
top (the OAV Onsen!)where Tenchi is tied to a 
cross with Ryoko and Washu guarding him. They end 
up having a plate spinning contest....it is 
just plain INSANE. :-) Someone on this board has a 
great cel of Ryoko and Washu with the spinning 
plates and a hell looking red/orange background, 
this is also from that episode.
Mihoshi is awesome, I would love a cel of her 
because she is decked out as Wolverine!
Somewhere I have the link to a website that has 
screen caps, I will try to find it for you guys.

And *maybe* your cel is familiar because it might 
have been used in ads for Cartoon Network Tenchi, 
I know they had scenes from this ep in their ads! 
Kathleen