Using Javascript to make a moving image

Hello!
I have a gif imge I made in Fireworks that I would like to use on my WordPress site. I would like this image to move from off the screen on the right, to the off screen of the left side. Basically moving from right to left across the screen. I would like this to be a single movement, no looping. This image should also start right when the user opens the index page, not on click or rollover. I am unsure how to do this. I am a student in web design so any input would be helpful.
Thank you so much!