Well you said "slide show" so I figured you were making some type of presentation. You can't make a scrolling text in a picture file unless you make an animated gif, but then it won't really scroll but jump from place to place. Maybe a good time to learn Flash, that is a good way to make a scrolling text over an image.
Maybe look into doing this as a presentation? OpenOffice has a PowerPoint type program you can use, free.