How to add new content as a background layer to an existing page?

Q: We want to integrate a background image (a logo) on every page at
its bottom. The logo shall always be behind the text.
In PDFNet there are several ways to append new content to existing
pages. Probably the simples approach is to use ElementWriter as
illustrated in the following FAQ:

Starting with PDFNet SDK v.3.6 ElementWriter.Begin() method has couple
of additional options:

- background: An optional flag indicating whether the new content
should be added as a foreground or background layer to the existing
page. If background is 'true', the graphics will be added in a
background layer. By default, the new content will appear on top of the
existing graphics.

- compress: An optional flag indicating whether the page content stream
should be compressed. This may be useful for debugging content streams.
Also some applications need to do a clear text search on strings in the
PDF files. By default, all content streams are compressed.

So, the simplest approach to add an image to an existing page as a
background layer is as follows:

ElementWriter writer = new ElementWriter();
ElementBuilder eb = new ElementBuilder();

// Create a new page content stream.
writer.Begin(old_page, true); // <-- 'true' means: add new content as
a background layer.
Image img = Image.Create(doc, "c:/peppers.jpg");
Element element = eb.CreateImage(img, new Matrix2D(200, 0, 0, 250, 50,
... add other background graphics