Hi Xiayao,
you can delete only a reference to the picture in the sheet with Sheet::removePictureByIndex().
For reading pictures please use Sheet::getPicture() and Sheet::pictureSize() methods.
Please note that the Sheet::getPicture() returns a global picture ID, so you need also pass it to the Book::getPicture() for getting a picture format and a pointer to the memory buffer with that image.
See also this example.
–Dmytro