Hmmm. Well, we could go really sophisticated like, "Welcome to the 2nd annual piano forum picture guessing event. If we have not received your portrait as of yet, please feel free to send it to *insert email,* with your user name attacherd, and we will be happy to include it. Have fun guessing which picture belongs to whom."
Or we could go more conventional like, "Welcome to our piano forum galary. Please join in the festivities by participating in "name to face" guessing game. Also, if you haven't submitted your picture as of yet, we always welcome more entries. Pictures may be sent, with user name attached to *insert email*."
Or we could go more game show like, "Welcome to our Guessing Game Galary! Would you like to know who these people are? So would we! Please contribute to the solving of the mystery by putting forth your guesses. And we always welcome more contestants, so please feel free to send in a smile (aka picture), with your user name attached *insert email*."
Or we could go more poetic like, "Wecome to our guessing game, all these faces have a name. Who is who we do not know, if you have an idea let us know! To extend the fun, please send your face, with user name attached, to this handy place *insert email.*"
The biggest victories are seldom posted on scoreboards.