Enable extensions in Safari 5 and download CloudPreview. Double-click on the file, and Safari will ask you to install this extension.
Once confirmed it should be available inside the extension list, and iTunes should no longer be opend automatically.
By default, this extension will resize really large images to fit into the browser window. You can disable this behaviour inside the settings.
You can download this project in either zip or tar formats.
You can also clone the project with Git by running:
$ git clone git://github.com/tobiastom/CloudPreview.safariextension