I don’t know, especially without seeing it. If it is the very top, above the neck, and a small crack, I wouldn’t be worried much myself. The stress is on the column below the neck. A professional like Carl would be much better to answer that.
A quick search shows three apps at the app store for resizing images:
Resize It
Image Resizer+
Desqueeze
I don’t use an iPhone, so I can’t recommend one over the other.