well I could not find it hear anywhere so I thought I would tell how to do it hear 
the default cubecap.png is in /usr/share/kubuntu-default-settings/kde4-profile/default/share/apps/kwin/cubecap.png
rename it to cubecap.png.old (with a root dolphin((kdesudo dolphin)) and copy your favourite .png in and rename it cubecap.png log out and back in and that’s it easy peesy
now the obligatory screen shot .............................

8)
☢VINNY☢

the default cubecap.png is in /usr/share/kubuntu-default-settings/kde4-profile/default/share/apps/kwin/cubecap.png
rename it to cubecap.png.old (with a root dolphin((kdesudo dolphin)) and copy your favourite .png in and rename it cubecap.png log out and back in and that’s it easy peesy

now the obligatory screen shot .............................

8)
☢VINNY☢
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