Yes, there is an image file in your root directory called default_image.gif which you can replace with your own image with the same name or you can open config.php and find this
Code:
# Default images
#
# The variable $default_image defines which image file should be used as the
# default "No image available" picture (a picture that will appear in the
# place of any missing image in your X-Cart-based store if no other "No image
# available"-type picture is defined for that case).
#
$default_image = "default_image.gif";
and change the variable to your own image