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You need to be more specific. A photo site like flickr or smugmug? Your own personal webspace? What exactly are you trying to do?
 
An automotive forum. I know that some sited allow you to upload from safari. browser. Flickr is another site i'm interested in uploading to.
 
As far as I can tell you, iPhoto only allows photos to be uploaded to MobileMe and Facebook.

If you want to use Flickr or ImageShack, you're going to have to copy those pictures to your desktop for easier upload. Or even finding a 3rd party app/plugin to do it for you.
 
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