15.6 Use png to be in sync with SLES

[lkocman]

• This allows re-running things that utilize default background such as cockpit. Unfortuantelly there is no unified location
• Use optipng -o5 to compress files

Driver for 15.6 would be https://jira.suse.com/browse/PED-7837, however I want to see full acceptance in Tumbleweed first.

[lkocman]

Leap 16 PR for reference https://github.com/openSUSE/branding/pull/147

[lkocman]

Oky Factory adopted png https://news.opensuse.org/2024/04/09/common-wallpaper-path/ 15.6 shall continue.