The final thing that worked was:
header('Pragma: public');
header('Cache-Control: max-age=86400');
header('Expires: '. gmdate('D, d M Y H:i:s GMT', time() + 86400));
header('Content-Type: image/png');
Now the browser does not make requests for the image when loading a page with embeded one.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…