Im not sure about this.
It is the files that Gameunreal said, but if you hex edit the system_plugin_bg.rco you can change the speed of the wave, width, amount of waves etc...
Yeah, its simple. Ive done it, so have lots of others.
You dont even need Photoshop to do it.
I wrote a simple guide on here somewhere but theres better ones to get an image into the wave ( i might remake it to get an image into the wave later)