these are probably some stupid noob questions, forgive me please if they have already been answered, I tried searching and didn't find anything related...

When I use the desktop cube, I get nothing but a black background (in the space around and behind the cube) My wallpaper on the cube is fine, the floor is nice and relfective, but no matter what image I choose in settings I get nothing but black... does the image file need to be stored in some particular place? Is it a glitch due to Nvidia Graphics card (I really don't have any serious problems with my Nvidia GeForce 7300 in any other way)

second stupid noob question, where do I find the image on the cube caps to change or replace it? I've searched all over the file system and can't find it... (it may not be where the usual placement would be since I added Kwin and some KDE stuff on top of an Lubuntu distro... would that alter the placement of the file?)

Other than that, I'm enormously pleased with Kwin... Compiz was nothing but trouble on my setup (lubuntu 13.04 running on an older Dell Vostro 200 with the Nvidia GeForce 7300 and 2 gigs of system RAM) I do find it a bit heavier than LXDE alone, but that's to be expected, and for a compositing window manager, the performance isn't too bad at all...

To enable caps, run "kwriteconfig --file kwinrc --group Effect-Cube --key Caps true"
Place a usable png at ~/.kde/share/apps/kwin/cubecap.png (might be ~/.kde4/... on your distro)

(IIRC it was "reserved" for distro branding, thus not configurable through GUI)

Then restart the compositor (press Alt+SHIFT+F12 twice)

About the cube wallpaper, i've no idea why that should not work, but try a different image (plain jpeg for the beginning, maybe 1024x1024 size) to ensure you're not just trying on a broken/unsupported image format.
When you're sure that this is not about the image file, please file a bug and add the output of "qdbus org.kde.kwin /KWin supportInformation"

thank you, both solutions worked perfectly

and the JPG for the BG didn't need to be 1024x1024, I used 1024x768 (approximately) and that worked fine, just had to be JPG and not PNG