Well, i signed up for a picasa web account, if i did make a custom tag it'd be kinda worthless imo, it'd require a user to post 90% of the path to the image, and they could do all that just with the image tag...
[picasa]moguai/RbCAWu1FEGI/AAAAAAAAAAo/uqyBEIFMAmE/413689385_l.jpg[/picasa]
-or-
I don't thin it'd save too much time or allow for anything that the current tools would allow...
Now to find some slideshow sites!