The OnMouseOver event (for example defined for a picture) can work even without a Javascript function, if we can stay with one Javascript command only, which can be written directly to the event handling.
In the following example, there are four small pictures in the upper line, followed by one big picture. The big one is changed if a small one is pointed by mouse. For the sake of making the task simpler, no small versions of pictures have been generated. Instead of this, the "width" parameter of pictures is used. As a result, initialization of page can take a while. On the opposite side of view, the image change will be very fast, because of all of the pictures have been already loaded in the browser.