I have both text and image in QLabel, so setPixmap won't satisfy my requirement.
As far as I know, QLabel can load image from file by setting HTML label <img src="path_to_file" />. But how can I load image from memory(e.g. QImage)?  Because some images are frequently used, it may have performance problem loading the same image from file every time.
You have two choices:

use two labels, one with the text and one with the image.
use a QPainter to draw the text over the image.
How to insert QImage(or sth like that) into QLabel?