Does anyone know the real names of the people who portraits the characters in the pictures?

no I don't.

I tried everywhere but couldn't find it.

Okay, i found a whole bunch of Pictures that are of the actors who portrayed the39clues but the names upon further investigation were typos and were the wrong actors. And the link to the names and the pictures was based entirely on some website that matches faces to actors, while the likeness was good, it was NOT the real actors.
If the authors are doing a signing in your area I'd snag the opertunity to ask them.