this code outputs pdf from html:
QTextDocument doc;
doc.setHtml("<h1>hello, I'm an head</h1>");
QPrinter printer;
printer.setOutputFileName("c:\\temp\\file.pdf");
printer.setOutputFormat(QPrinter::PdfFormat);
doc.print(&printer);
printer.newPage();
I guess you can generate an html wrapper for your img and quickly print your image. Otherwise you might copy the image directly on the printer, since it is a paint device in a similar fashion
QPrinter printer;
QPainter painter(&printer);
printer.setOutputFileName("c:\\temp\\file.pdf");
printer.setOutputFormat(QPrinter::PdfFormat);
painter.drawImage(QRect(0,0,100,100), <QImage loaded from your file>);
printer.newPage();