Problems with the images of the products by the backoffice, show only "?"

[diegousm]

people

 

I have the following problem, when I enter the store by the backoffice and enter in catalog> products, the miniature images of the products are not displayed and the sign "?" Is attached, I attach an image if it is not clear. (Delete the name of the products)

 

 

How can I recover the images, if someone knows how to fix the problem please help me.

 

for the front office I can see perfectly the images of all products, is prestashop 1.6.

Edited November 29, 2016 by diegousm (see edit history)