If you are hosted with Hostgator, you may have a problem with the automatic thumbnail generation feature that many WordPress themes provide. To solve this:
1. Login to your Hostgator control panel and contact Live Chat Support.
2. Ask the support technician to white list your thumbnail generation file for mod_security . You will need to give your file name and path. The file name mostly will be timthumb.php . You can find this file inside the WordPress theme folder. An example file path wp-content/themes/theme_name/timthumb.php
This is a known issue with Hostgator’s WordPress hosting and all the technicians can recognize your request. This problem will be solved by them in minutes.