PKG installer - part of background image is hidden - installation

Question

I'm trying to set a background in .pkg file.
Here's my distribution.xml template file:
<?xml version="1.0" encoding="utf-8"?>
<installer-gui-script minSpecVersion="1">
<title>{{ title }}</title>
<welcome file="welcome.txt" mime-type="text/plain" />
<readme file="readme.txt" mime-type="text/plain" />
<license file="license.txt" mime-type="text/plain" />
<conclusion file="conclusion.txt" mime-type="text/plain" />
<pkg-ref id="{{ identifier }}"/>
<options customize="never" require-scripts="false"/>
<background file="{{ background_filename }}" mime-type="image/{{ background_ext }}"/>
<choices-outline>
<line choice="default">
<line choice="{{ identifier }}"/>
</line>
</choices-outline>
<choice id="default"/>
<choice id="{{ identifier }}" visible="false">
<pkg-ref id="{{ identifier }}"/>
</choice>
<pkg-ref id="{{ identifier }}" version="{{ version }}" onConclusion="none">{{ identifier }}-{{ build_number }}.pkg</pkg-ref>
</installer-gui-script>
It will be rendered from a Python script, and called with productbuild:
background_path = os.path.join(
config_dir,
config['darwin']['installer']['background_path']
)
background_filename = os.path.basename(background_path)
distribution_rendered = self.source.render_template(
self.__osname,
"productbuild/distribution.jinja2",
{
"title": ' '.join(
(
brand_name,
binary_name,
version
)
),
"background_filename": background_filename,
"background_ext": os.path.splitext(background_filename)[1][1:],
"identifier": identifier,
"build_number": build_number,
"version": version
}
)
with open('distribution.xml', 'w') as f:
f.write(distribution_rendered)
os.mkdir('resources')
shutil.copy(background_path, 'resources')
runcmd(("productbuild --distribution distribution.xml "
"--resources resources "
"--package-path {0}.pkg compiled/{0}.pkg").format(filename))
The .pkg file was created successfully except that part of the background image is hidden:
How can I make this transparent, so I can see the hidden part of the backgroud image?