install the default to gd wordpress plugin
https://github.com/alhoseany/default-to-gd As of WordPress 3.5, GD has been abstracted out into WP_Image_Editor, and Imagick has been made the default image manipulation engine.

In most cases, this is what you want.  However, if you’re having issues with Imagick, or just prefer the way that GD handles your images, this plugin sets GD as the default WP_Image_Editor class for WordPress.

other fixes:

1- change permission on async-upload.php

usually this is done when the normal browser uploader is working and the multiple file uploader or the flash uploader is not working

chmod 644 /public_html/wp-admin/async-upload.php

2- htaccess solution in wp-admin folder

# Exclude the file upload and WP CRON scripts from authentication
<FilesMatch "(async-upload.php|wp-cron.php|xmlrpc.php)$">
    Satisfy Any
    Order allow,deny
    Allow from all
    Deny from none
</FilesMatch>
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>

3- htaccess above the web root or in “public_html”

<IfModule mod_security.c>
  <Files async-upload.php>
    SecFilterEngine Off
    SecFilterScanPOST Off
  </Files>
</IfModule>