Here are a couple of tidbits you’ll want to know before beaming photos from iPhoto to Flickr.
Getting set up. The easiest way to start setting up Flickr is to attempt to share a photo with the service: select a photo, choose Share > Flickr, and you see this dialog.
Click the Set Up button, and iPhoto switches you to your browser, which opens the Yahoo sign-in page. (Flickr is owned by Yahoo.) After you sign in, follow the instructions to authorize the iPhoto uploader. You know you’ve reached the right place when you see a big, friendly OK, I’ll Authorize It button.
Click this button, and another Flickr page notifies ...