I've created this.

(it's 5 mB)

The cover and the separators in the pages are generated stretching on layers the same image (a pdf generated by Metapost)

% cover

\setlayer [mybg]

[x=0\paperwidth, y=0\paperheight]

{\color[lightgray]\externalfigure[alternando]

[height=1\paperheight,width=1\paperwidth]}

% red horizontal separator

\setlayer [mybg2]

[x=0\paperwidth, y=.45\paperheight]

{\color[darkred]\externalfigure[alternando]

[height=.1\paperheight,width=\paperwidth]}

% gray vertical separator

\setlayer [mybg3]

[x=.9\paperwidth, y=0\paperheight]

{\color[lightgray]\externalfigure[alternando]

[height=\paperheight,width=.05\paperwidth]}

In Preview.app on macosx it's all displayed as expected 

(this is a png snapshot of the horizontal separator: 

 ). 

 But Acrobat displays the separators (the cover is ok) as a sequence of filled boxes. Can it depends on some resolutions setting? Or what?

-a-