Google Operating System blog tells:
Picasa Web Albums offers more free storage if you are a Google+ user. According to Picasa Web’s help center, “photos up to 2048 x 2048 pixels and videos up to 15 minutes won’t count towards your free storage”. If you upload photos from Google+, they’re automatically resized to 2048 pixels on their longest edge, so they don’t use the 1 GB of free storage that’s available in Picasa Web Albums.
For those of you not on Google+ yet, only pictures up to 800×800 pixels won’t count up on your storage. You really should let me know your email address and I’ll send you an invite.