I like your 2nd idea :), but i don’t know if it’s suitable enough for the contest.
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“Your Mission
Create an iconic image that captures a moment of galactic civilisation or conflict.” and “Think big really, really big! This is your opportunity to create an iconic image that captures a pivotal moment in a vast galactic spanning civilisation”
I don’t think your group of explorers and your huge alien (:)) will fit with the contest. I might be wrong tho. Maybe if you alter your idea a bit, like instead of explorers you could use a large group of people colonising a new world, not aware of the alien presence. It makes the concept a bit bigger, make them flee in terror from your big aliens, completely surprised by their presence. Obviously they’re bad and willing to kill ;). Just an idea. I might be wrong tho… if i am, ignore everything i said, but it would be a damn shame if you’d work out a cool concept that doesnt fit with the contest.