I didn't think people would seriously be discussing this. My take:
They very well could be positioned at varying elevated levels as you suggested--especially since these guys can fly and stand on nothing but air. The fact that we can't see the lower half of this pic means its almost pointless to get disappointed over the supposed discrepancy regarding Gohans height in relation to Goku. Also, for the sake of the promo pic, it's likely that the person who illustrated this wanted Goku positioned slightly higher than everyone else to emphasize his role as main protagonist and father/leader of those two.
Either way, its too early to call it a retcon, contradiction, mistake, or whatever else comes to mind based on that picture alone.
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