Hello and welcome.
If you downloaded the picasa.dmg file directly from Google, then it is safe to open. If you downloaded it from a third-party site, make sure it is one that you have used previously and trust. That is just the new standard warning dialog to verify that you downloaded it and that you know where it came from.
As Spawn_Dooley referred to, Picasa is slowly being retired, and I wouldn't count on much further development.
C