Hi Florent. I did some extensive tests on your server with 4 images:
manual.jpg - resized to 1600px from my desktop. Not resized on upload.
resizer_yours.jpeg - resized to 1600px from your resizer uploader.
resizer_mine.jpg - resized to 1600px from MY local panel resizer upload.
resizer_yours_lowq.jpeg - resized to 1600px but with very low compression.
First of all, all checks show that there is no difference in these images on the threshold of what your server manages to resize. Your limited server manages to serve this specific image at 768px width. (I found this exact value by test-and-fail to see at exactly what number your server fails).
http://www.bsbd.fr/render/w768-q90/temp/manual.jpg
http://www.bsbd.fr/render/w768-q90/temp ... r_mine.jpg
http://www.bsbd.fr/render/w768-q90/temp ... yours.jpeg
http://www.bsbd.fr/render/w768-q90/temp ... _lowq.jpeg
All images above 768 fail:
http://www.bsbd.fr/render/w769-q90/temp/manual.jpg
http://www.bsbd.fr/render/w769-q90/temp ... r_mine.jpg
http://www.bsbd.fr/render/w769-q90/temp ... yours.jpeg
http://www.bsbd.fr/render/w769-q90/temp ... _lowq.jpeg
So first of all, let me just conclude that your server is very limited. It says 256MB in your
php.ini diagnostics, but it seems there is a virtual limit set to
24117248 bytes (25 MB).
Also, we can rule out that there is anything different in your uploader than mine, as
resizer_mine.jpg vs
resizer_yours.jpeg is same. We can also rule out that is is related to image file size and/or quality, as the same behavior is exhibited on resizer_yours_lowq.jpeg
So what is the difference between the images you uploaded?
This is the question that puzzles me also. Did you upload by FTP? What application do you use for resizing? I can only imagine two scenarios:
1) For some reason, your server dismisses the virtual limitation if your files were uploaded by FTP user. This could be related to security.
2) Your locally resized images have some super-powered stripping features that lowers the memory consumption when resizing images.
Only your server
I also tested all images locally, and this only happens on your server ... I also have 256MB in my testing environment, and all images fail at some point, but they are always the same between images resized from the uploader, and images resized manually.
This is why I lean towards scenario
#1, which would explain everything. According to the output from the images that fail, you have
24117248 bytes limitation. If that is so, then your manual resized images should fail also ... but they don't. Why don't they fail? There
must be a difference in the memory provided to resize the different images.
As you can clearly see in my tests, there isn't any difference whatsoever in a file uploaded with- or without resizing. If you give me FTP, I could test that.