Update preview images for new "Flow" wallpaper

Summary: Depends on D29449

Test Plan:
{F8288340}

{F8288339}

Reviewers: #plasma, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29468