They seem vaguely humanoid but it's pretty much impossible to determine what they look like just from this picture. The Earth Surrenders newspaper shows what might be these humanoid alien soldiers, at the bottom-left corner of the main picture in front of a Strider. They appear to be wielding weapons as they march to battle to support the Striders. The Vortigaunt mural depicting the Combine invasion of the Vort world features a group of vaguely humanoid figures being deployed from a dropship container very similar to the one seen in Half-Life 2.
Half-Life: Alyx shows two possible instances of humanoid alien Combine soldiers that may or may not be Synthetic, though they could just be cyborgs like their human counterparts. They might have them considering that Vortigaunts and other species in the Nihilanth's army are humanoid themselves.
