When dual-booting Linux with Windows 7, Picasa will not recognize mounted EXT2, EXT3 or EXT4 partitions under Windows. (First of course, you need to get windows to recognise the partitions with the EXT2Fsd drivers.)
To add EXT4 (etc) partitions to Picasa under windows:
Mount your EXT2, EXT3 or Ext 4 partition under windows.
Network share the partition.
Browse your network shares and Map a drive letter to the shared partition.
Add the Network Share to Picasa.