How do you convert a powerpoint ppt doent to bmp or gif?

Hi there. I wanted to make a picture collage to put as my desktop wallpaper. I didn't know any other way to do this (I'm a little computer illiterate) and so I made one using powerpoint. Now I'm having a dilemma on using this as the wallpaper because its not compatible. Can someone please tell me how I can covert the files? Or if its easier to do a picture collage some other way, could you please tell me? Thanks so much in advance!!