You mean you want to manually take a picture and use that instead of a webcam?
If so then yes you can do that, take the picture, save it as a .jpg file (in a suitable image size, like 320 by 240) on your WD PC.
Then follow the instructions here http://discourse.weather-watch.com/t/12283 using the .jpg you saved as the “webcam file source” (box 1 on the first big screen shot on that page).
When you want to replace the picture just save the new one in the same place with the same filename and WD will use that one instead.