just recently, yahoo posted a site and some tools to help the transition go quickly and smoothly.
i personally only have about 2 dozen yahoo pictures in the system, but thought it would be good to move them anyway to flickr (where i already have an account and another couple of dozen pictures). you can also push your images to shutterfly or snapfish.
if you have any images in your yahoo photo “drawer” you should start looking at moving them to a new service sooner rather than later. i opted for the move, yahoo photos sent an email saying that the move had started and they would notify me when the images had their new home with flickr.
moving forward there are some tools you can tie into iphoto to easily and quickly export into flickr. check out connectedflow’s flickrexport. you can also go to flickr’s site and just download their tool for bulk import of images into flickr – check out the options.