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 recognise your request. This problem will be solved by them in minutes.
Before approaching them, make sure that all folders’ permission is set to 755 and files’ permission is set to 644. You may also be asked for the credit card / PayPal info to verify that you are the owner of the hosting account.