help! How do i make an image float accross the screen? Read more for details.

[Rubiano]

How do i make an image of my choice float across the screen. I want an image, the size of say a smilie, to float across the screen on my forum. I want to pick when it floats across the screen too. I want to make it start at maybe like 4:00 PM and float across the screen every 15 minutes. How do i do this? I have seen it on another vBulletin forum.(www.webkinzinsider.com/forum). Please explain in detail, how i can do this. (I am a beginner.)