I want to do a mail merge and post out a letter to members of a Civigroup. You can do this by generating a message template. When you insert a letterhead image into the template, the image resolution on the printed copy is poor. Is there a way to improve the resolution so that it looks professional. I presume this would be the same with invoices etc.
If the printed version looks poor it is probably due to the quality of the image you used in the editor. You could simply upload one with better quality / resolution.
In general you may have to fiddle a bit to get your html templates work well. I would advise to use an external editor and backup your templates once they work.