I've reviewed the documents you have sent.
Even though there was no final document sent to you, the fact that the document speaks to the terms of the agreement, states that the parties have agreed to the terms, and is signed by both of you is sufficient to form a contract.
The long and short of it is that there is no period of time which obligates Koo to return the painting to you. Thus, you will need to formally terminate the agreement and demand return of the unsold art.
You should start by sending a formal letter, delivered by certified mail return receipt requested, or by Federal Express with delivery confirmation. In the letter you need to state:
I am writing to provide you with formal notice that the Art Consignment Agreement between us is terminated as of the date of this correspondence.
I have received commissions on the sale of the following pieces:
Thus, you are still in possession
I demand that you immediately ship the remaining pieces listed above to me at _____________________.
Because you have failed to keep me informed of your progress of the sale of the art I consigned to you, and because you have refused to be forthright with me in the communication and accounting regarding the consignment, I am forced to demand that you return these pieces at your own expense within 30 days of your receipt of this letter.
Should you fail to do so, I will have no other choice but to initiate legal proceedings against you.
Signed ( by you)
As you can see, your only true course of action is to make a formal demand and then if it is not complied with, you must initiate litigation
The fact that you signed the contract in Nebraska is not likely going to be good enough to establish jurisdiction over Koo in the courts of Nebraska. You could attempt it, but might face a motion to dismiss
for lack of personal jurisdiction. This would add to your legal costs in the end.
Rather than file the suit in Nebraska, it is better to file the suit in the state where she lives. That way you know that there will be jurisdiction over her, and you will likely only have to travel to the state on 2 occasions (one for a deposition if requested, and two for the trial if reached and not settled). A high percentage of litigation settles.
Because you are talking about some very valuable property, you would need to hire an attorney to handle this matter for you.
Please let me know if you have any questions.