Error message

• Warning: imagegif(/data/vhosts/schoolcat/drupal7/sites/default/files/styles/medium/public/covers/9780618259892_0.jpg): failed to open stream: No space left on device in image_gd_save() (line 318 of /data/vhosts/schoolcat/drupal7/modules/system/image.gd.inc).
• Warning: imagegif(/data/vhosts/schoolcat/drupal7/sites/default/files/styles/medium/public/covers/9780785121442_0.jpg): failed to open stream: No space left on device in image_gd_save() (line 318 of /data/vhosts/schoolcat/drupal7/modules/system/image.gd.inc).
• Warning: imagegif(/data/vhosts/schoolcat/drupal7/sites/default/files/styles/medium/public/covers/9781607531913_0.jpg): failed to open stream: No space left on device in image_gd_save() (line 318 of /data/vhosts/schoolcat/drupal7/modules/system/image.gd.inc).