Ugh, that sounds like an awful situation, Lil.
I honestly have no idea what the best course of action is. It's easy to write what to do, but in person I'm more of an avoid-conflict-at-all-cost, let-it-simmer-for-years, then-explode kind of zebra. You've already brought it up, so part of me thinks pushing the issue may cause more defensiveness/avoidance at Lucy's end. OTOH, you're understandably hurt (and concerned) and letting it fester probably won't end well either.
If it was a one-off, it'd be easier to write off as an oversight or a bit of thoughtlessness, but it sounds like this is repeated behaviour. Which is concerning. And also, can I just say for the record what a dick move on Lucy's part?
How good is the mutual friend who posted the pics? Like, would she realize how it'd hurt you, or would she be unaware? (Basically, is she someone you could use as a third-party sounding board about the situation?)