A few things to keep in mind when creating Wallpapers in CorelDRAW.
1) All Drop Shadows or Glows, are created at the DPI of your screen.
2) Any bitmaps you make as part of the image will have there own DPI.
3) Any Bitmap effects will have there own DPI, this included Fractals and Transparency Masks using bitmaps and fractals.
You will have to play with it to see the what I mean, but I find that I get the best results for wallpapers if I fallow the same guidelines I use for exporting Icons.
1) Set up you page to be 300dpi.
2) Design the wall at 1" = 100 pixels. Example most the time I do walls at 12.80" x 10.24".
3) Export the Image as a Transparent PNG, at 300dpi. Then Resample it down with some other graphics program.
I have a exporting Video over in the CorelDRAW for skinners tutorials, you might want to check it out.