Its working fine on a sample server, everything. But on a new install at different server, the thumbnails are not loading, not a single one. Asked the host about server logs, all clear and no other problem. If you think you can do the job, please PM me urgently. It'd be a minor fix though.
ImageMagick or GD2 might not be installed on your server. Ask your server support to install it for you.
@dpacmittal - You are right with your knowledge and quite often its that issue. However, this time it was different. The problem actually was the host shared wrong info, that the mod_security is edited for a specific file, while it was not. All sorted out and resolved. Thanks to everyone who replied and PM'ed. @guardian999 - Yes. @moderator - This thread maybe closed and mark as resolved please, if possible. Thanks.