Really no idea why there is a transparent bikini on the two scans I found, reminds me of post #2307352 where they needlessly made the girls transparent or something.

I think they are supposed to be those type of glossy bathroom posters where certain parts become transparent when wet.

Or body paint.

One of two pin-ups drawn by Happoubi Jin to commemorate 10 years of Comic Megastore. The other one was with Mika :

http://twitpic.com/c9938h
and some better sample :
post #1395738

Ha, it's even on sale :
http://www.suruga-ya.jp/product/detail/608919837001

The current one :
http://www.suruga-ya.jp/product/detail/608912255001