When would you want to make a single image not selectable? This doesn't seem super useful. Even if there is a use case, it is likely only useful in the setup mode. In animate mode you can click an image to select bones now, but I'm not sure disabling that for a single image is useful.
Since it isn't the most important feature and I just want to get it done, create a backgrounds node and all that isn't appealing. We could have a selectable and export checkbox for images, but only if there is a good reason to have 2 checkboxes instead of 1.