How to automatically rename image uploads? How to automatically rename the image files to a random string during the upload process? For example, I have an image named "big_poster.jpg" and I want it to be saved as something random, for example "2680542675157_cc7m541v_l.jpg" 

I see every asset have a "Filename" and "Title" field. Would be possible to use these fields to generate a random string? Perhaps using the Randomm plugin?
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