Press the “Add widgets” button on the top right. PDF-header itself acts in fact as a set of widgets
Select “Image upload” option in following menu and press “Next”
Drag & Drop file with your logo to the highlighted area or browse for it and press “Apply”.
Click cogwheel icon in the File upload widget top right corner to edit uploaded image if needed. You can add or remove captions, resize your image, select placement (on the top or the bottom of the page), return to widget setup or delete this image at all.
Return to your report with “back to report” button on the left or create a PDF with “Export to PDF” at the top right corner. Anyway, now “export to PDF” will automatically add selected image on every page of resulting PDF-file.