Three new, action packed pictures have been released by Blizzard. Two of them feature the Zerg, which we haven’t seen or heard of*in a long while.

Unfortunately, the pictures only feature very few Zerg unit types. A couple of things do stand out, though:
Overlord?
The hideous flying monster in this picture is probably none other than the new version of the Overlord.

The Overlord of StarCraft 1 is an important unit that serves many roles for the Zerg:

• Flying detector
• Unit carrier
• Unit control (raises maximum supply)

It’s not clear why an overlord is present in the attack depicted in the picture. The attacking Zerglings and Banelings are attacking the heavily defended Terran choke point - all non-stealthed targets. The Overlord might be attempting to drop a few more Zerglings behind the choke point, perhaps to deal with the Siege Tank that’s situated on the ridge near it. Of course, the new Overlord could have a completely new ability - one that might have something to do with the glowing green orbs it has on both sides of its… head.
If the unit itself doesn’t convince you, take a closer look at its shadow, which resembles the old Overlord even more.
Dark Swarm?
A weird looking orange effect is clearly visible on top of the swarm of Zerglings. While the only Zerg units present in this picture are obvious non spell-casters (Zerlings, Banelings and Mutalisks), the effect is most likely a spell.
Dark Swarm, a Defiler spell, protects units under its area of effect from ranged attacks. This is the perfect protection for a tight group of swarming Zerg units, which would otherwise suffer heavy damage from Terran splash-damage attacks by Siege Tanks and Thors. Take a look at the original spell from StarCraft 1 for comparison:

Also worth noting in this picture is the huge Thor that’s acting more as a gatekeeper than a siege unit on top of the defended ramp. The Thor is a heavily armored unit with plenty of hit points that will likely be resistant to attacks by small units which deal low amounts of damage, like Zerglings, making it a good choice for this role. Likewise, the exploding Banelings are more suited to deal with clumps of weaker targets than large armored ones.

After a long Zerg-free period, we’re glad to see any Zerg presence in StarCraft 2 updates. Hopefully, these pictures are just the first trickle of information about the Zerg race, which we still know virtually nothing about.
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From the Starcraft 2 Blog.