My photographer's standard contract asserts ownership of all the images she creates, and allows me to create prints for personal use. But she also says she will delete all images 3 months after supplying them to me electronically. I will hold the only copies. Does she still own them even though she chose to discard them?
She would still own copyright, yes.
See 17 USC 302(a):
Copyright in a work created on or after January 1, 1978, subsists from its creation and, except as provided by the following subsections, endures for a term consisting of the life of the author and 70 years after the author’s death.
Nothing in the following subsections cuts short the duration of copyright upon destruction of an original.