Well, I just took a quick look at the client, and I think I know how to get you going. You're going to start by reading my tutorial on how make graphical edits to Conquer Online's client:
From there, you'll find out that some of the files in the Conquer Online client needs to be decompressed from a file to be accessed. The file you'll need to decompress (using the guide I linked you to) is data.wdf. Once you extract those files, you'll want to go to data/main. That's where most of the interface graphics are. Conquer Online cycles through a few login screen graphics. They're named "bg#" (the number being the number of the background). Switch those out and you'll be good to go. You don't need any fancy editor. They're just jpegs I believe. The characters on the other hand are a bit more to edit. You'll need something like Paint.net to edit them. Cheers.