1. enable [multilib] repository.

2. Install winetricks, wine, and lib32-lcms from multilib.
# pacman -S wine winetricks lib32-lcms

3. Then download and install Picasa:

wget dl.google.com/picasa/picasa39-setup.exe
export WINEARCH=win32
winetricks ie7
wine picasa39-setup.exe

Now you got picasa working.
Next is installing video codecs so Picasa can pick up .MOV files, etc.
Download this codec pack, and install with Wine. Select "Advanced" during install, and make sure you never select the "Windows" options when you can avoid them.
http://codecguide.com/download_k-lite_codec_pack_basic.htm