Lots of legal issues today...
I am a lawyer. Let me say to start with, that each state's procedural rules vary, so I can't give a definite answer. Also, I have to give the disclaimer that by offering the following few general comments, I am not attempting to give you legal advice or otherwise establish an attorney-client relationship.
With that out of the way, in general, a third party (that is, someone who is not a party to the lawsuit) is not under any obligation to give any kind of documents, including photos, to a party in a lawsuit or their attorneys. Nor are you required to talk to the parties or their attorneys, even if you are a witness...
...BUT, that changes if the lawyer asks the court for a subpoena. A subpoena is an order from the court requiring you to provide copies of the documents or photos to the parties, or to appear at a deposition (where you would give sworn testimony in response to questions). If you receive a subpoena, you are required to give the parties the documents, or else you can be held in contempt of court and fined or even imprisoned.
Now, the party seeking the subpoena has to pay you your expenses that you will incur in complying with the subpoena. Ordinarily, that means paying your gas money and mileage to appear at a deposition, or paying the copying costs for providing the documents they requested. For pics, that would mean strictly the cost of getting copies made of the photos. It would NOT be some amount along the lines of a royalty you would charge someone ordinarily to buy the rights to your photo.
My guess is, this is how this situation will play out. If you offer to sell the photos to the lawyer, s/he will likely refuse, because (a) they can subpoena them later anyway, and (b) another lawyer may be able to put doubt in a jury's mind about the veracity of evidence that the lawyer purchased. S/he will then get a subpoena from the court, and you will be required to provide the photos, and you will get the costs of producing the copies, but nothing else.
I could be wrong, but that is how I would anticipate things to go, unless the lawyer just decides that the pics aren't necessary.