Picasa Web Albums has been upgraded to Google+ Photos. all Photos from Picasa Web have been Synchronized with Google+ Photos.
You can sign-in with a standard Google Account or Google Apps Account.
You can also Customize Picasa Slideshows and Add Picasa Photo Gallery to Google Maps/Panoramio.
A CDN [Content Delivery Network] like Picasa Web albums/Google+ Photos will help increase your sites web performance.
Picasa Gallery for the URL.
You can create a gallery browser fullscreen link for you slideshow.
Copy the link in your picasa account.