Yaranaika is from Kuso Miso Technique, a yaoi manga which starts with a guy named Masaki getting hit on by a strange man (Takakazu Abe) in a jumpsuit while walking through a park.
"Yaranaika" translates as "Shall we do it?" and is Jumpsuit Guy's de facto catchphrase, and "Uho Ii otoko!" ('Whoa, nice guy!' or more loosely "Hey, good lookin'!") is the first thing he says to Masaki.
One of the manga's most notable features is the highly exaggerated look of the protagonist's faces, especially while having sex, which made it a huge hit on 2chan. Masaki's and Jumpsuit Guy's faces (both normal and in various states of sexual arousal) were shopped onto just about everything imaginable. This, of course, spread to /b/ and became popular there as well.