Thanks sbilliard. I've checked individual files permissions in zp-core, it's all set to 644.
The server I use actually propose a module which automatically install an older version of zenphoto and which I used to run before (I may rollback to that previous version if I we can't solve this problem) so I doubt it has something to do with a functionality at the server level, unless you've completely changed the architecture of the program with version 1.3, which I don't think you did.
I've enable DEBUG_IMAGE (not in folder-definition.php but in basic-function.php) and have played a little bit with zenphoto and raise one or two errors. Don't see error messages in it, might be meaningful to you?
{Wed, 22 Dec 2010 21:21:49 GMT} Zenphoto v1.3.1.2[5831]
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
i.php(test4sbillard, img_4192.jpg): $size=400, $width=, $height=, $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $thumbstandin=, $passedWM=, $adminrequest=1, $gray=0
cacheImage($imgfile=img_4192.jpg, $newfilename=/test4sbillard/img_4192_400_admin.jpg, $allow_watermark=, $force_cache=, $theme=zenpage) $size=400, $width=, $height=, $cw=, $ch=, $cx=NULL, $cy=NULL, $quality=100, $thumb=, $crop= $image_use_side=longest; $upscale=0;
propSizes(400, , , 853, 1280, , longest, 267, 600)::$neww=267; $newh=400
cacheImage:img_4192.jpg: $size=400, $width=, $height=, $w=853; $h=1280; $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $newh=400, $neww=267, $dim=400, $ratio_in=, $ratio_out= $upscale=0 $rotate= $force_cache= $grayscale=0
propSizes(400, , , 853, 1280, , longest, 267, 600)::$neww=267; $newh=400
cacheImage:no crop img_4192.jpg:$size=400, $width=, $height=, $dim=400, $neww=267; $newh=400; $quality=100, $thumb=, $crop=, $rotate=; $allowscale=1;
Finished:img_4192.jpg
Loading PHP GD library bundled (2.0.34 compatible)
i.php(test4sbillard, img_4192.jpg): $size=400, $width=, $height=, $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $thumbstandin=, $passedWM=, $adminrequest=1, $gray=0
cacheImage($imgfile=img_4192.jpg, $newfilename=/test4sbillard/img_4192_400_admin.jpg, $allow_watermark=, $force_cache=, $theme=zenpage) $size=400, $width=, $height=, $cw=, $ch=, $cx=NULL, $cy=NULL, $quality=100, $thumb=, $crop= $image_use_side=longest; $upscale=0;
propSizes(400, , , 853, 1280, , longest, 267, 600)::$neww=267; $newh=400
cacheImage:img_4192.jpg: $size=400, $width=, $height=, $w=853; $h=1280; $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $newh=400, $neww=267, $dim=400, $ratio_in=, $ratio_out= $upscale=0 $rotate= $force_cache= $grayscale=0
propSizes(400, , , 853, 1280, , longest, 267, 600)::$neww=267; $newh=400
cacheImage:no crop img_4192.jpg:$size=400, $width=, $height=, $dim=400, $neww=267; $newh=400; $quality=100, $thumb=, $crop=, $rotate=; $allowscale=1;
Finished:img_4192.jpg
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
i.php(test4sbillard, img_4192.jpg): $size=800, $width=, $height=, $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $thumbstandin=, $passedWM=, $adminrequest=, $gray=0
cacheImage($imgfile=img_4192.jpg, $newfilename=/test4sbillard/img_4192_800.jpg, $allow_watermark=1, $force_cache=, $theme=zenpage) $size=800, $width=, $height=, $cw=, $ch=, $cx=NULL, $cy=NULL, $quality=100, $thumb=, $crop= $image_use_side=longest; $upscale=0;
propSizes(800, , , 853, 1280, , longest, 533, 1200)::$neww=533; $newh=800
cacheImage:img_4192.jpg: $size=800, $width=, $height=, $w=853; $h=1280; $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $newh=800, $neww=533, $dim=800, $ratio_in=, $ratio_out= $upscale=0 $rotate= $force_cache= $grayscale=0
propSizes(800, , , 853, 1280, , longest, 533, 1200)::$neww=533; $newh=800
cacheImage:no crop img_4192.jpg:$size=800, $width=, $height=, $dim=800, $neww=533; $newh=800; $quality=100, $thumb=, $crop=, $rotate=; $allowscale=1;
Finished:img_4192.jpg
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)
i.php(test4sbillard, img_4192.jpg): $size=400, $width=, $height=, $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $thumbstandin=, $passedWM=, $adminrequest=1, $gray=0
cacheImage($imgfile=img_4192.jpg, $newfilename=/test4sbillard/img_4192_400_admin.jpg, $allow_watermark=, $force_cache=, $theme=zenpage) $size=400, $width=, $height=, $cw=, $ch=, $cx=NULL, $cy=NULL, $quality=100, $thumb=, $crop= $image_use_side=longest; $upscale=0;
propSizes(400, , , 853, 1280, , longest, 267, 600)::$neww=267; $newh=400
cacheImage:img_4192.jpg: $size=400, $width=, $height=, $w=853; $h=1280; $cw=, $ch=, $cx=, $cy=, $quality=100, $thumb=, $crop=, $newh=400, $neww=267, $dim=400, $ratio_in=, $ratio_out= $upscale=0 $rotate= $force_cache= $grayscale=0
propSizes(400, , , 853, 1280, , longest, 267, 600)::$neww=267; $newh=400
cacheImage:no crop img_4192.jpg:$size=400, $width=, $height=, $dim=400, $neww=267; $newh=400; $quality=100, $thumb=, $crop=, $rotate=; $allowscale=1;
Finished:img_4192.jpg
Loading PHP GD library bundled (2.0.34 compatible)
Loading PHP GD library bundled (2.0.34 compatible)