change variable according to onmouseover event

Hello, i wonder how can i achieve the following.

i want to load a page , and make for example a 800x800 sized part of the page where i want to load 1 image  , BUT onmouseover that image, other images would load.

So basicly here is what i want:

<div>

<a href="" onMouseOver="value=true"> </a></div>

and somewhere down on the page , i would like to check this value , and if it is true than do something, or else, do other stuff...Without refreshing the page of course... I know this can only be achieved with javascript, but i've no clue how...

call a function from your mouse over event

<a href="#" onmouseover="change_image(true);">...
.... later in your script
function change_image(val){
    if (val) // change image here
    else // change image here
}