Image tag src-attribute: Session lost with mobile connection

I use an <img> tag with an src-attribute pointing to a script that loads the image from a non-public folder. When I test my website with a mobile connection, the scr-attributes path is changed from src=" to src=", I guess that has something to do with my mobile internet provider. The problem is, the image-providing script cannot identify the session of the incoming request anymore.
On a non-mobile WIFI connection, the src-attribute is not changed, everything works as expected and the session is found.

Source: stackoverflow-php