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You can either use the built-in WordPress plugin install menu, or do it the old-fashioned way:

1. Unzip the .zip archive and put the imageshack-offloader folder into your plugins folder (/wp-content/plugins/).
2. Activate the plugin from the Plugins menu.
3. Go to Settings -> Imageshack Offloader and customize your settings.

Usage

The plugin will automatically filter the post content and replace image src attributes with ImageShack URLs.

If you want to get the URL for a specific image, you can use get_imageshack_url($id, $size), where $id is the attachment id and $size is the desired size. Default is 'full'. If there is no URL yet, the function will return false.