For people searching, the question might also be phrased: How do I restore Picasa albums from backup? I need to get the original, high resolution file back, as Picassa reduced the file size form 4MB to 1.5MB.
When I reinstalled my computer and restored my photos from backup, some of my albums showed up, but many didn't.
I've found the following info:
Picasa on Windows stores album info in these places:
Vista: C:\Users\<myaccount>\AppData\Local\Google\Picasa2Albums\ XP: C:\Documents and Settings\<myaccount>\Local Settings\Application Data\google\Picasa2Albums\
I restored that folder and was still missing many of my albums. That folder also contained a folder of backups, but the most recent one was from a long time ago and I've created albums since then.
According to https://support.google.com/picasa/bin/picasa.google.com/support/bin/static.py?hl=en&page=release_notes.cs, since the Dec 8, 2011 build, Picasa saves album info in
.ini file(s). This probably explains the albums that I do see.
http://katelharrison.blogspot.com/2012/01/how-to-restore-picasa-albums-mac.html has some great info on restoring albums on Macs, but the folder structure seems to be different there than on Windows.