Picasa 2 seems to work fine with the latest version of Crossover running on a Mac. The only error I've noticed so far is that the displayed text labels in the software are all pixellated - I'm guessing they're some sort of font that isn't supported in OSX.
The slideshow for some reason doesn't display full screen.
Web functions are fine, email functions are fine (even with Google Mail)
I'm happy having just left my PC behind (maybe forever now!!)
Thanks