The image argument is passed when the plugin is called from the UI, not when the plugin is registered. If you have 10 images open, the plugin can be applied to each image by activating a view on the image (image windows or tabs) and then calling the plugin from the menu or a shortcut. Likewise the plugin automatically sets the "drawable" argument to the current active drawable of the current image.
However, an external process cannot determine the current image, if this is your problem (and anyway it could have changed in between....). So you probably want to send the image ID to your external process with the rest of the data so that it can use it to retrieve the image for which it was called later.
However, an external process cannot determine the current image, if this is your problem (and anyway it could have changed in between....). So you probably want to send the image ID to your external process with the rest of the data so that it can use it to retrieve the image for which it was called later.