[2013]Quick Question - Replacing Art Files

It's been a while since I've posted here. Yes, I'm still doing 2013 while y'all have moved on to 2014.
Just a quick newb question. How does one replace the arts of cards, while retaining the original art files? I gather you'd have to create new TDX files with the appropriate names, but where should I put them? In a separate WAD? A folder in an existing WAD? How do I make sure that the game reads the new arts and not the original ones?
Just a quick newb question. How does one replace the arts of cards, while retaining the original art files? I gather you'd have to create new TDX files with the appropriate names, but where should I put them? In a separate WAD? A folder in an existing WAD? How do I make sure that the game reads the new arts and not the original ones?