How could I do this?

I'm trying to replace the advertisements on various billboards in Liberty City. But I cannot seem to find out where I change them. I have found the ad I wish to change as my first one because it is easy to find around the map, just a short drive to the Airport. I export the texture but then, I cannot open it as it's in a dds file. I took a picture of it with snipping tool and edited it. I have what I want it to be. Now how do I change that picture I edited into the thingo to support gta iv & how do I import it into the game via OpenIV. As I can only find the .wdr file and I can only view that. There is the .wtd files but it says that there is no Images in it.

Cheers for your help!

Edited December 20, 201510 yr by claw106