Flickr is owned by Yahoo! now, so it will default to your yahoo account when you connect. You may have to logout first to get to another account.

Help:
There are tools for Windows and the Mac to make uploading easier:
  see Flickr / Help / Tools: Tools to upload and share

Flickr Tour

Links:
Flickr vs Picasa - Pocket-lint
Flickr and Picasa - Easy Photo Sharing