This is happening to me, too: I’m getting error messages similar to the one posted above.
Warning: imagecreatefromjpeg() [function.imagecreatefromjpeg]: ‘/home/sista25/public_html/wp-content/uploads/1231.jpg’ is not a valid JPEG file in /home/sista25/public_html/wp-admin/admin-functions.php on line 724
Warning: imageantialias(): supplied argument is not a valid Image resource in /home/sista25/public_html/wp-admin/admin-functions.php on line 731
Warning: Cannot modify header information – headers already sent by (output started at /home/sista25/public_html/wp-admin/admin-functions.php:724) in /home/sista25/public_html/wp-includes/pluggable-functions.php on line 272
I have plenty of disk space. It’s not entirely random — some image files work fine, but certain ones don’t.
For the ones that won’t work, I checked the images on the server. The actual uploaded image is there and can be viewed, but the thumbnail is black.