Look, the Organa's knew Padme and probably saw no harm in telling Leia about her. As a consequence, she probably had a picture in her mind of what Padme was like.
The Lars knew Padme and that if they told Luke what really happened to her than his safety would be compromised.
So Leia's memory could have come from the Organa's...anyway, it's a good of enough reason for me.