please go through here: http://www.prestashop.com/forums/topic/327240-product-images-have-been-replaced-with-marks/
this worked me!
It looks like a problem with rewriting the image urls. To be sure that your .htaccess file is being regenerated I would delete (or rename) the current .htaccess file in the root directory and then try again to generate a new .htacces file by disabling friendly urls - save - and enabling friendly urls and save. Then you can be sure to have a brand new .htaccess file. Maybe worth a try ...