Have you reviewed the troubleshooting guide? http://www.zenphoto.org/2007/12/troubleshooting-zenphoto/
Warning: imagecreatefromjpeg() [function.imagecreatefromjpeg]: gd-jpeg: JPEG library reports unrecoverable error: in /home/percep/public_html/gallery/zp-core/functions-image.php on line 61
Warning: imagecreatefromjpeg() [function.imagecreatefromjpeg]: '/home/percep/public_html/gallery/albums/still-life/IMG_1790.JPG' is not a valid JPEG file in /home/percep/public_html/gallery/zp-core/functions-image.php on line 61
Warning: Cannot modify header information - headers already sent by (output started at /home/percep/public_html/gallery/zp-core/functions-image.php:61) in /home/percep/public_html/gallery/zp-core/i.php on line 134
The first error is the only one of consequence. The second is a direct result of the error message display from the first error.
The error says that the GD handling routines did not like the the JPEG image that was passed to the function. In this case, the image is the one that is being processed for caching after resizing.
As to what the routines did not like, there is no information. But this might indicate a corrupted image.
Yes, please see here: http://www.zenphoto.org/2008/08/troubleshooting-zenphoto/#11