http://inaturalist.org/picasa/options
You can also get there by editing your account settings and clicking
the "Link or unlink your Picasa account" link. It's been working
pretty well for me, but please drop a line if you find something
broken. I've also found that Picasa seems to preserve EXIF metadata a
lot better when you upload using their desktop client instead of
uploading directly to the Picasa website. Also, if you add EXIF
geotags to your photos, make sure you've configured Picasa to
automatically read location data from the photos you upload at
http://picasaweb.google.com/lh/settings#permissions. Otherwise iNat
won't be able to import the location data.
-Ken-ichi