QImage和QPixmap转Base64字符串

    QImage img(path);
    QByteArray ba;
    QBuffer buf(&ba);
    buf.open(QIODevice::WriteOnly);   //需要加这句才能成功！！！

    img.save(&buf, "jpeg");
    QByteArray ba2 = ba.toBase64();
    QString b64str = QString::fromLatin1(ba2);

参考于：Qt QImage和Base64之间的转换 - 仆人 - 博客园 (cnblogs.com)