This and the Mahoromatic picture are front and back of a hard, laminated card that measures about 18cm x 26cm. If anybody knows what they're used for, could you plz let me know ^_^

What you have is a small pencil board, or shitajiki - popular in schools as a portable 'hard surface' to write on, they are frequently illustrated with anime and manga characters. The size you have is a medium, and is probably designed to fit in a notebook. There are about 3 or 4 different sizes available. Punch up Shitajiki on Wikipedia for a full explination!

A-N

11months late there, hehe. :)
Also: what's up with the dot on Rin's face?

cute pic though - but it seems a little blurry

Usually pencil boards are like that. The texture just makes linearts look thin.