First you have to upload company logo. For this:
From Basic->Setup->Company informations->Company logo upload your logo.
After that write code below which I found from this address as display method:
Display Bitmap CompanyLogo()
CompanyInfo companyInfo = CompanyInfo::find();
return CompanyImage::find(companyInfo.DataAreaId, companyInfo.TableId, companyInfo.RecId).Image;
Pull this method to header. Don't forget arrange Height and Width values manual and change ResizeBitmap property to Yes.