Now I have made my changes, but I see it's a difficult system. You use frontend template (made by me) backend template (Joomla) and also (via Joomla i think) system templatestyles. So I have to work on several files and hope that changes wont be eraised when Joomla is updated. Would be nuicer to have a more efficient templating system.
Now I'v finished my invoice, people can print it, but Images are not shown (logo f.e.). And I dont see because of that complex templating where and in with stylesheet or file I now have to change something.