simple coding problem

[Jshortest]

Hey I'm doing a gallery and when you click a pic it will display it in a box in the top right. When no pic is clicked I have an image i made saying no image to display. how can i make something to say this

<?
If 
$pic=" " then $pic=('nopic.jpg')

[laserlight]

Well, one way would be to use a name/value pair in the query string to pass which pic has been selected.
So
page.php?pic=1

would mean you should display pic #1

In that case, you just need to determine is $_GET['pic'] is empty or not:

//array of pic numbers corresponding to pic filenames
$pic_array (
	'1' => 'apple.jpg',
	'2' => 'balloon.jpg',
	'3' => 'cat.png'
	);

//check if $_GET['pic'] is empty, or if an incorrect value entered
if (empty($_GET['pic']) || !array_key_exists($_GET['pic'], $pic_array)) {
	//display no pic
}
else {
	//display pic based on corresponding $pic_array element
}