Does anyone know where the game's graffiti textures are stored? I'm working on higher res versions and I found the ones in "Textures\Clutter\Graffiti" but I know there are more, like a graffiti heart, stick figures, etc. I feel like I've looked almost everywhere but haven't turned up anything.

I'd do it this way:
In the GECK locate the cell where is your desired graffiti. Remember the *.nif file and extract it with BSA unpacker. Then open the *.nif in NifSkope and locate the node where is the texture reference.

I'm not a pro by any means so there should be another way(s).