Merging an image into header of each related list table
Insert a header or footer into a document, then merge fields and images into the header/footer. You can also insert an image merge field or any merge field into the top of a table. Note: there is not a way to make the table header repeat on each page if the related list repeats multiple rows/values.
One thing to consider as possible option is allowing the page to break, then merge other tables filtering on the quote line items. For example, calculating the characters or lines in the description field and using that to try to group the line items into pages and merging them in groups for each page.