I'am big fan of the "Centaurea" wallpaper by Kasprzak Micha, also because if matches the blue color scheme of KDE so nicely.

For those how want it to match another color scheme, Centaurea-fs is the solution: Here the flower is translucent, so you can choose the color of the flower by setting the desktop background color.
It doesn't look quite as good as the original, but that's the price for the increased flexibility. It's your choice