The first images are blank

The url is
http://cmsdev.xxx31.fr/plugins/skinfactory/modele-desxxx31-1/_thumbnail/32/32/model/variants/webfont1-css/thumbnail_32.png
(the red thing was for a test to check that I have done the modification with paint.net)

Note that if I replace by thumbnail_32_photo.png it works. If I copy and paste the content of the "bad" image into "photo" and save with paint.net, it works. If I take this picture, send it to https://tinypng.com/ then it does not work.

If I take the original image, save it with paint.net or with https://tinypng.com/ (which is a reference) the image can be displayed directly in a browser but not thru the URL of skinfactory.

Note that If i change the url http://cmsdev.xxx31.fr/plugins/skinfactory/modele-desxxx31-1/_thumbnail/16/16/model/variants/webfont1-css/thumbnail_32.png then it works

In my case there are a lot of images not working on 4.4 but working on 4.3.