Ok, world building doesn't lay rivers in any predictable fashion. Rivers do not seem to flow towards progressively lower elevation. Rather, they seem to choose an acceptable ending point (glacier, lake, ocean) for each source and random-walk their way until they hit it, another ending point, or another river (and merge). This is rather frustrating if you're trying to build a geography conducive to particular river features, because that geography is completely irrelevant.
Try pre eroding your bit maps and see what happens with rivers. It is a bit random, but generally I can make a river go wherever I want it on a map. After you import your bitmap, don't use Perfect World's elevation generation AT ALL, turn both noise sliders down to zero so there is no randomness from Perfect World's side of things. There still will be some randomization from DF's world gen, generating the same world off the same map will yield slightly different results, but if you pre-erode your river system into the landscape as deep valleys using Wilbur, the randomization from DF won't change the course of your rivers much. Like I said, it makes very realistic looking worlds, with hills in between the rivers, forest surrounding the smaller ones, and marshes and swamps surrounding the larger ones. Mountains ranges look more realistic, too, with spurs and valleys that look, well, eroded, with many branching valleys.
It takes a bit of work, I spend about half an hour drawing, eroding and tweaking my bitmaps before I even import them into Perfect World, but you absolutely CAN get the terrain features you want that way. Just make sure you are working with the full range of contrast in your graphics editor or paint program, oceans should be pure black and mountain peaks should be pure white.
(EDITED) My latest bitmap for use in Perfect world. I edited it at 4x size, 1028x1028, then shrunk it down to 257x257
You aren't going to get great big waterfalls off of this one directly, but there are several points you could easily edit to create some really big ones.