I see from the comments that we have two issues:
- What spell is Voldemort using, and
- if it is Avada Kedavra, how can Harry block it?
Clearly I am writing out of pure speculation:
I feel that the spell used is most definitely Avada Kedavra, mainly because of the signature green flash, the fact that Voldemort uses it almost all the time, and also given the situation: hell broke loose and everyone has chosen to fight - Voldemort got no reason to subjugate/harm professors or students that oppose him. He needs to dispose of them. Oh and also the fact that this is a movie adaptation: I'm sure the director would realize that Voldemort using a green spell that is NOT Avada Kedavra would be very confusing to the audience.
Said that, it boils down to the next question: how can Harry block this supposed Avada Kedavra? The comments insist that Why is Avada Kedavra considered unblockable? shows how is this possible, but the accepted answer does not apply to this context: Harry is not using a physical object, nor is Priori Incantatem possible in this scene and he is definitely not under Lily's protection either. He is obviously using a spell.
My guess is that it is the same reason why Harry's Expelliarmus surpassed Voldemort's Avada Kedavra in the final showdown: Voldemort's wand is not his. The Elder Wand's true master is Harry and hence it will refuse to kill Harry, who happens to be in the middle of the attack. Avada Kedravra was powerless against Expelliarmus in the final scene, and by that fact I conclude the Avada Kedavra used in OP's picture was also powerless against whatever defensive spell Harry used: the Elder Wand will not kill its true master.
There is an obvious flaw with the above speculation. Voldemort should realize that something is wrong. He used Avada Kedavra and Harry blocked it! Why would he continue fighting? This may very well prove that the above conclusions are wrong, or the solution is something much simpler: Voldemort is proud and arrogant and there is no way he is going to flee when the whole crowd is watching him and Harry circle each other to kill.
That's my two cents.