IMO I don't think the picture was that bad, there are many worse photos out there. Like the OP has said pervs will be pervs. She obviously has a good relationship with her daughter, she is friends with her on the social media pages so is aware of what is being posted.
OP, I am not sure what her settings are like on social media but it could be worth checking and making sure that perhaps they are set to private so that on her friends can see what she is posting? I know that doesn't change what she is posting but at least you will know exactly who can and can't see it.
Also we are all only really seeing half the post, I think also the 'caption' or the hashtags to the image will provide more context to the picture - sometimes those things sway which was a photo is perceived / interpreted by friends / family etc
Such a hard one with social media these days. I would just keep an eye on her accounts and keep communicating with her and making her aware of the perceptions images can have - you sound like you are doing a great job so far!