In Powerpoint 2007 you can:
1) right click on the individual slide.
2) select 'Save as picture' in the context menu that appears
3) in the 'save' dialog box, select an image type (.jpg, png, gif or whatvere options are available)
4) browse to the folder you want to save to.
5) give the file a name.
Then if you have any image editing software, crop/edit and optimise your image file for the web.
Then add it to your web page as any other image.