Su-cool is short for Sunao Cool, and is a character type like tsundere originating from Futaba's rakubaki-ban board: http://up.2chan.net/q/futaba.htm. The first su-cool image is post #32726 by Komiya (c-ko).
Su-cool girls are blunt and rarely get emotional. They confess their love early, in public if need be, and remain cool-headed, never blushing or displaying embarassment.
Artwork featuring su-cool girls usually feature an unambiguous love confession.