Something fun for anyone into graphical stuff.
Go to your urbanterror/q3ut4 and extract the textures folder from zpak000_assets.pk3. Have a look through it for something that looks familiar and copy it to somewhere safe, like your desktop. Load it into gimp and mess around with it. Now, on your desktop, recreate the folder structure that your original image was in and put your newly modified image in there. Zip these folders up and rename the resulting zip to a pk3 with a name starting with zz, like zz_newstuff.pk3
If you put this pk3 in urbanterror/q3ut4 it will override the existing pk3s (as long as it's name starts with zz) and the image inside it will replace the original in your map.
Here's an example pk3
This is how it looks in the game.