The second picture seems to be a bit more significant to me as it shows the Chevy Volt parked with other Autobots. Last time we saw the Volt was at the Queensway Bridge set (here and here) which was being used as a Shanghai, China site. At the time a GM rep said the car is not a Transformer alt mode but its looking pretty clear that is not the case.
The suggestion was the car is essentially an extra used for a scene at the Queensway, but if that was the case why spend the time and money to transport the car from California to New Mexico and in the movie storyline from China to Egypt (or vice versa since not sure what comes before what in the story)? The way I see it, this pretty much confirms that the Chevy Volt is an alt mode for un-identified Autobot Transformer. Considering the Volt is part of "green" friendly push by GM, I highly doubt the car is a Decepticon. As usual this is all speculation but it fits the concept car M.O. established by the first movie and continued in this one.