How to reproduce the error.

1. Create a new node and add an image in the /node/add/story form

* The node is created fine with the image.

2. Press the "Back button" from your browser (FF in my case) and return to the last page /node/add/story

3. Change the title and the body and create a new node without any image.

Now a appear and error because node_images are trying to add the last image, and now there are not any image.

This is the error message:

The selected file %file could not be copied, because no file by that name exists. Please check that you supplied the correct filename.

Unable to copy image "XXXXXX.jpg".

# notice: Trying to get property of non-object in .../sites/all/modules/node_images/node_images.module on line 493.
# notice: Trying to get property of non-object in .../all/modules/node_images/node_images.module on line 494.

The node is created but there are and invisible image in the Images tab.