When you check both "Stretch image to use as cover background" and "Resize cover dimensions to match background image" options in the Dimensions tab, this error is generated:
calibre, version 0.9.6
ERROR: Unhandled exception: <b>IOError</b>:[Errno 2] No such file or directory: u'bookcover_01.jpg'
Traceback (most recent call last):
File "calibre_plugins.generate_cover.dialogs", line 1510, in options_changed
File "calibre_plugins.generate_cover.dialogs", line 1446, in update_current_options
File "calibre_plugins.generate_cover.draw", line 26, in get_image_size
File "site-packages\calibre\utils\magick\__init__.py", line 146, in open
IOError: [Errno 2] No such file or directory: u'bookcover_01.jpg'
It seems the problem is there is a stray "u" added before the image filename, which produces the "no such file or directory" error.