hmm, I dunno, It probably depends on what sort of paint you're using, but in my experience acrylic doesn't really soak in that much. (then again I always put on multiple layers no matter what I am painting on.) If you're referring to "Depersonalization," that was actually painted on the back of two vinyl record sleeves that I unfolded and stapled together, so it's a different quality of cardboard.
I lay them on my floor to photograph them, always in natural light with no flash, (I can only take my photos during the day) then I just crop the photos so no carpet is showing, and edit the contrast a little in Picasa.
flashes create glare (obviously) and I find that artificial light distorts the colours.