Picasa Web Albums allows you to upload photos from Picasa to the web. By default, an album is public, so everyone will be able to see it if he knows the homepage of your Picasa Web Albums. By default, the homepage is picasaweb.google.com/gmailusername, but you can change the name of the directory. You can also make an album unlisted, so it won't be included on the homepage, but anyone can find it in the Google search results (if you or someone else links to it) or can try to guess its name. This means there's a good chance that someone interested in your pictures will be able to find them if he tries hard enough.
A sign that the unlisted albums are about to become private is that Google includes a parameter in the URL of unlisted albums: authkey, followed by a security code. There's also a message that warns people to update the bookmarks with the new URL.
It's interesting to see if the unlisted albums will be available to anyone who knows the URL or they'll require invitations.
Interesting features in Picasa 2.5 Beta
Picasa Web Albums uploader for Mac