Just use var_dump() to see what's in $_POST :

var_dump($_POST);

And you'll see that, when your form is submitted using the <input type="image">, you get :

array
  'buyuka_x' => string '0' (length=1)
  'buyuka_y' => string '0' (length=1)

So, there is no $_POST['buyuka'] -- instead, there are :

• $_POST['buyuka_x']
• and $_POST['buyuka_y']

Which means your code should look like this (not testing for the unexistant buyuka entry, and testing for the two _x and _y -- I suppose that testing for one of those should be enough) :

if(isset($_POST['buyuka_x'], $_POST['buyuka_y']))
{
    $sorgu='SELECT * FROM urunler ORDER BY uyeno DESC';
}

Edit after the comments : I have no idea why it goes like that -- but having a .x and a .y is how it's defined in the HTML standard.

If you take a look at Forms in HTML documents, and scroll down a little, you'll be able to read :

When a pointing device is used to click on the image, the form is submitted and the click coordinates passed to the server.
The x value is measured in pixels from the left of the image, and the y value in pixels from the top of the image.
The submitted data includes name.x=x-value and name.y=y-value where "name" is the value of the name attribute, and x-value and y-value are the x and y coordinate values, respectively.

In PHP, the dots in parameters names are automatically replaced by and unerscore.
So :

• name.x becomes name_x
• and name.y becomes name_y

As a source for that last statement, you can read Variables From External Sources - HTML Forms (GET and POST) (quoting) :

Dots and spaces in variable names are converted to underscores.
For example <input name="a.b" /> becomes $_REQUEST["a_b"].