Thank you to take the time for the reply. I did consult the troubleshooting guide more than once, but guess I missed the essentials

. I did read lots of postings from the forum too.
And sorry I did not expressed myself clearly about fault tolerance, I meant more fault tolerant to the zenphoto user standpoint. I finally understood my issue was specific to GD, and GD have an explanation about unsupported jpg headers between softwares, etc. From the front-end, I am still a zenphoto user finding a way to get my images displayed.
But as I know my images are not "that bad", all I did was rotating them, so I choose to temporarily ignore the warnings. Even though it is not the best solution, it's good to know that such a workaround could help
Will try to install Imagick and see if there's a difference in behaviour.
Thank you for your support.