Rounding up humans from Earth, the monstrous aliens drove them in chains across the landscape, enslaving them for labour to restore their abandoned munitions factories. The humans later escaped, due to exploiting the crippling weakness the aliens had for stuffed parsnips.
In this painting, the aliens actually belong to two different races within the species. The trigger-happy alien on the right holds a Brain Fusion Gun, which may be conveniently abbreviated to BFG.

Man, I hate taking digital photos of my art, but it was necessary for this one, as it was too big, even to piece together in my scanner, due to the lid of the scanner being attached to the longer side of the scanner. So here we have it, like a grainy distorted version of the truth!