Qt uses lossy encoding when writing images to a PDF file. I believe it forces JPG encoding, even if this is grossly inappropriate. This can lead to bad distortion of images in the output file and artefacts.
This can mostly easily be seen when writing low pixel count images to a PDF file.
Please provide an option to disable the lossy JPEG output and use PNG/Zlib instead.
Attached is an example which uses a simple 3x3 image showing large differences from the also-written PNG file.