Thanks!:) specifically, the location for the cubecap image is at: /usr/share/apps/kwin/cubecap.png.
I assume you create a new image, give it the same name, and replace it in that directory. I'll give it a try!:)
Thanks for posting, that's all I needed to go into folder as root and put my own like size .png file in place and name it cubecap, just saved other as .old
simplicity is nice