There are some third party Picasa apps available (mostly paid if you want a good one) that work well with Picasa web albums. You can use Picasa's mobile site via the browser for uploads. Flickr has an iPhone app that works on the iPad but is optimized for the smaller iPhone screen. Works though for uploads.
I started using Photobucket to move pictures between the iPad and other devices. Dropbox also allows uploads of multiple pictures but not downloads to the iPad of multiple pictures. You could also check out Apple's iPhoto, which I believe has Dropbox integration. I have not used it though so I can't give it a recommendation for the $5 price tag. I bet you can find tons of reviews on it though.
Good luck! Getting photos on and off of the iPad has proven exceedingly difficult for me as well. It seems this should be easy but things get a bit too restrictive for my taste here with the iPad. Perhaps this will change in future iOS releases.