default image wont display consistently

[concession-supply.com]

default image wont display consistently.
When the product does not have any images configured on the product page, why does the default image sometimes display and sometimes it doesn't? What is the correct default behavior for products that are missing thumbnail and product images? :confused:
And, if the behavior to call up images in the common folder, we need to alter it. Our default images have been overwritten again.
Thanks in advance! :)