Nudity/sexually explicit acts. Nothing illegal or particularly extreme. My concern is with losing my job and source of income as a result of those photos being shared, not the photos themselves as much.

You obviously know the company/situation better than me, but I think it's pretty unlikely that they'd sack you.

File a police report first, and then take the whole thing (including the police report, and maybe the photos, if you have them) to your boss. Explain that you're really embarrassed, but you're being blackmailed for something that is NOT illegal, that the police are involved, and that the blackmailer has threatened to send the photos to your employer. Ask them to just let you and the police know if they hear from her.

Then tell her you've told your employer already, and she's got no leverage left.

I wouldn't tell her about the police: if they end up charging her (which hopefully they do) she'll hear about it soon enough, and better it comes from them than you.

Good luck!