Aha, just done some investigation and looks like while aasimar tend to have at most feathers where wings would sprout, third level aasimar get a "transformation" ability they can use for a minute per long rest such as "Radiant Soul. Starting at 3rd level, you can use your action to unleash the divine energy within yourself, causing your eyes to glimmer and two luminous, incorporeal wings to sprout from your back." or "Necrotic Shroud. Starting at 3rd level, you can use your action to unleash the divine energy within yourself, causing your eyes to turn into pools of darkness and two skeletal, ghostly, flightless wings to sprout from your back.". The third option doesn't seem to include wings: "Radiant Consumption. Starting at 3rd level, you can use your action to unleash the divine energy within yourself, causing a searing light to radiate from you, pour out of your eyes and mouth, and threaten to char you." The pics I've spotted are presumably of aasimar who have activated their transformation ability and I've just assumed they're always there! Still, looks like the ability to show wings of some sort would be a prerequisite for having aasimar in game, but I was right about that for the wrong reasons so don't really deserve credit for it
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