I'm not lawyer but once wanted to be one
A contract does not have to signed, witnessed or in writing to be valid. The key to any contract (verbal or written) is 'consideration'. In essence, consideration is that for doing or providing x I give you y.
The 'contract' as shown is, in my opinion, absolute rubbish and would have no chance of any enforcement. The key aspect is that there is no delivery date or defined product other than the term 'image'.
As there is no defined delivery date and as I would have taken exception to her texting 'not negotiable' when expecting me to give up a Sunday then I would:
a. run this by a lawyer or at least CAB
b. tell her that you have got advice and are getting further advice and that nothing will be delivered until you have had that advice
c. tell her that there is no date by which you must supply the images in the contract and that regardless of what she may want you have no intention of delivering anything until ordered to do so by a court or advised to do so by your solicitor.
I would also advise you to stop thinking of her as a client. Until she parts with some money she is someone who wants your work but is not prepared to pay for it. That is not a client.
I would not delete the images as it is possible that you will be advised (when you get some professional advice) to hand them over. I cannot see this happening but it is possible that a court would view the fact that she paid for the studio time as 'consideration' on her part and that the written contract has no standing but the implied or verbal contract is still valid.
You are talented and your talents will grow. She is good looking but her looks will fade. Learn from this and move on.
John