It all depenends on what you want to do with the image. Is it going to be a full screen background?
Or will it be a smaller image along with text or titles?
Or do you just want a part of the picture that you have downloaded?
"WebShots" has several neat options for downloading, they even have a 16:9 aspect ratio which is great if your PowerPoint presentation is created in 16:9.
Just download the format you want and in the PowerPoint slide that you want the image to be. Chose: insert/picture/from file.
which will place the image in your slide, or for a background image choose: format/background/fill effects/picture.
Paul