Error Uploading Avatar

[OakleyForum]

Sever log is filled with this error, and members are saying they can't upload avatars:

Any idea whats causing it?

ErrorException: exec() has been disabled for security reasons - library/XenForo/Upload.php:197
Generated By: racs09, 14 minutes ago
Stack Trace
#0 [internal function]: XenForo_Application::handlePhpError(2, 'exec() has been...', '/home/oakleyfo/...', 197, Array)
#1 /home/oakleyfo/public_html/library/XenForo/Upload.php(197): exec('/usr/bin/mogrif...')
#2 /home/oakleyfo/public_html/library/XenForo/Upload.php(281): XenForo_Upload->_checkImageState()
#3 /home/oakleyfo/public_html/library/XenForo/Upload.php(154): XenForo_Upload->_checkForErrors()
#4 /home/oakleyfo/public_html/library/XenForo/Model/Avatar.php(48): XenForo_Upload->isValid()
#5 /home/oakleyfo/public_html/library/XenForo/ControllerPublic/Account.php(482): XenForo_Model_Avatar->uploadAvatar(Object(XenForo_Upload), 4806, Array)
#6 /home/oakleyfo/public_html/library/XenForo/FrontController.php(310): XenForo_ControllerPublic_Account->actionAvatarUpload()
#7 /home/oakleyfo/public_html/library/XenForo/FrontController.php(132): XenForo_FrontController->dispatch(Object(XenForo_RouteMatch))
#8 /home/oakleyfo/public_html/index.php(13): XenForo_FrontController->run()
#9 {main}
Request State

 

[OakleyForum]

Fixed it by reuploading upload.php