How to automatically rename image uploads?
Use the assets.onBeforeSaveAsset event
You could write a plugin that listens for when an asset is saved via the assets.onBeforeSaveAsset event and rename the image before it is saved to the database?
So you could have something like this in your plugins init method:
craft()-&gt;on('assets.onBeforeSaveAsset', function(Event $event) {

    // Only rename if it’s a new asset being saved
    if($event-&gt;params[‘isNewAsset’])
    {
        $asset = $event-&gt;params[‘asset’];

        $asset-&gt;setAttribute(‘filename’, ‘SOME_RANDOM_STRING’);
    }
}

I haven’t tested this code, but from looking at Craft’s source and the docs this should work.
Documentation about events can be found on the Craft website