I would say that if you are in any doubt at all, give the Abbott customer services a ring, and offer to send them the pic through email, so they can make an informed decision.
- don't email them before speaking to them though, I have read several times that emails don't get fast responses whereas the phone is a much better way to contact them.
Having said that, if the sensor is working correctly, and you have no swelling or sign of infection, then you could just wait and see what happens.
I've never had a bruise like that, but then my arm has more... er... padding... than yours, so the veins are less accessible.
Maybe just make a mental note not to use that square centimetre of arm for future sensors.