When page starts to load, it creates session and then output is generated afterwards. File imgstore.php displays images in order to prevent unauthorized access. However it is executed somewhere in the middle of the code. The problem is that imgstore.php also calls session_start(), but since we already have output, it fails and can't display image properly.
If we try to open the imgstore.php, we can see why:

session_start(): Cannot send session cache limiter - headers already sent (output started at C:\Development\ZBX-8950-test\frontends\php\include\classes\html\CObject.php:46) [imgstore.php:99 ? session_start() in C:\Development\ZBX-8950-test\frontends\php\imgstore.php:99]