Migratr is a no-frills Windows utility that allows you to bulk
Related: Download Flickr Photos in a Batch
You select a source (like Flickr or Picasa) and every picture from your account is downloaded locally. The next half of the cycle will upload them to the destination. All the important information including picture title, description and tags are preserved during the transition.
If you have an account of two or more photo sharing services, Migratr is a good tool to consolidate them into one place. Or you can create backups of Flickr on Picasa and vice-versa. No support for Photobucket or Imageshack.
Migratr [Upload Flickr Images to/from PicasaWeb Albums]