I understand your questions, and I want you to know that typically the format of this site is chat, in that you ask questions, I ask follow up questions to better get an idea of your situation, etc... Surveillance laws tend to be fact dependent, which is why I wanted to chat with you, rather than this current format. However, I will attempt to answer these questions with the information that you have provided me.
In pennsylvania is it legal for an employee to take photographs of me during a private meting without my knowledge or consent? - Most likely not. It actually depends on whether or not there is a "reasonable expectation of privacy". For instance, it's not illegal to take pictures of people on the street without their knowledge, even though it might be invasive. But you can't take pictures of people in a bathroom without knowledge or consent. These are the extremes of "reasonable expectation of privacy". In a prosecution for illegal video, photo, or audio surveillance, the prosecution would have to establish that there was a "reasonable expectation of privacy" to show that the surveillance was illegal. If you have an open office layout, then there might not be such a reasonable expectation of privacy. But if it's a closed door meeting, and the photos / videos came from a concealed / hidden camera, without your knowledge or consent, then that would be illegal in Pennsylvania.
In pennsylvania, is it legal for an employee to record a private conversation in my office without my consent of knowledge? - No, most likely. Again, just like above, it depends on whether there was a reasonable expectation of privacy in that situation. If you have a cubicle or open office setting, then it would be much more difficult to get a conviction for illegal recording. But if it was a closed situation, and the employee did not tell you of the recording and you did not give a blanket consent, then that would be illegal.
This employee is also slandering me to HR with complete fabrications after she was asked to submit her resignation. - Slander is actionable civilly (in civil courts, rather than criminal ones) if you can prove a false statement of "material fact", to a third party, that has caused quantifiable harm. It has to be a material fact stated (in that it can't just be an opinion), which is false, and has to result in quantifiable harm. That means that you could calculate the damages, to a reasonable degree of certainty. If you were fired from your job because of the defamation
, you could sue because you could quantify the lost wages, and possibly the damage to your reputation on top of that (based upon difficulty in finding a new job, etc...). If you have not suffered any clear quantifiable damages, but only potential, or ONLY reputational, then slander would not be actionable (as outrageous and egregious as the lies may be, and unethical and immoral as the actions of this employee were). But if you can actually quantify the damage that you have suffered, then you could sue that employee for the defamation. Note that quantifiable damages are a necessary element to prove that cause of action.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, XXXXX XXXXX luck to you!