Configuration Settings

Right, so I have started to make my first module. And after scouring the internet I am stuck. It's to do with the Admin Configuration Page of my module. I have successfully displayed a form, but I am very unsure of how exactly to process the form. I'm new to Drupal, but I know it won't be as simple as just using the standard if(isset($_POST)) etc. etc...I expect there is some "hook" or something similar that I don't understand completely that I need to put in...

My code for the form is as follows:

function randomimage_admin() {


  $form['upload'] = array(
    '#type' => 'fieldset',
    '#title' => t('Remove Images'),
    '#tree' => TRUE,
  );
  
  $preamble = 'Your current photos are listed below. To remove a photo, ';
  $preamble .= 'check its box and press submit.<br/>';
  
  $form ['upload']['preamble'] = array(
    '#value' => $preamble,
  );
  
  // Now retrieve all the images being used
  $result = db_query("SELECT * FROM {randomimage}");
  
  while ($pic = db_fetch_object($result)) {
  
    // Reset the value of the prefix and suffix
    $prefix = '';
	$suffix = '';
  
	if (!isset($i)) {
	  $prefix = '<table><tr>';
	  $i = 0;
	}
	
	elseif($i == 0) {
	  $prefix .= '<tr><td colspan="4">&nbsp;</td></tr><tr>';
	}
	
	$prefix .= '<td class="randomimage" ';
	$prefix .= 'style="background-image: url(\'/sites/default/files/images/';
	$prefix .= $pic->filename;
	$prefix .= '\')">';
	
	$suffix = '</td><td>&nbsp;</td>';
	
	$i++;
	
	if ($i == 4) {
	  $suffix .= '</tr>';
	  $i = 0;
	}
	
    $form['upload'][$pic->iid] = array(
	  '#type' => 'checkbox',
	  '#title' => '',
	  '#prefix' => $prefix,
	  '#suffix' => $suffix,
    );
	
  }
  
  if ($i != 4) {
    $end = '</tr><tr><td>&nbsp;</td></tr>';
  }
  
  $end .= '</table>';
  
  $form['upload']['images'] = array(
    '#value' => $end,
  );
  
  return system_settings_form($form);
}

So what steps do I take to process the form data?

Thanks
Jack