Problem with change image format

[ChrisW67]

But I want an image to grayscale representation of 8 bits (256 colors).

Try building a 256 element grey colour table and then call the variant of QImage::convertToFormat() that takes a colour table and build an indexed image that way.

If you want to do it manually then look at qGray(). Your simple average of R, G, and B values is not a good reflection of the perceived brightness of colours.