Here you need to create new system parameter, in that give key as a ‘webkit_path’ and value of wkhtmltopdf (webkit executable) which you need to download from Google Wkhtmltopdf Project.
After set the Parameter you can set the header and image. Go throw Settings > Companies select your company than set the header following step: Webkit > Header/images.you can select and create the header for your company.
After set the header go to the account menu. Customer > Customer invoices. You can see in print menu webkit invoice.