How can I get image of configurable product. I found this code but it returns null
$_swatchUrl = Mage::helper('configurableswatches/productimg');
The methods available in the helper you mentioned allows you to retrieve the image urls for the configurable swatches. For example the url of pink/white/blue swatch url for a super attribute such as color (ex: media/catalog/swatches/1/21x21/media/pink.png).
You can check in
.../configurable/swatches.phtml how is being used
by the core:
$_swatchUrl = Mage::helper('configurableswatches/productimg')
->getSwatchUrl($_product, $_option->label, $_swatchInnerWidth, $_swatchInnerHeight, $_swatchType);
But you want to take the image of a parent/configurable product so just follow the code from
src="<?php echo $this->helper('catalog/image')->init($_product, 'image') ?>"
which is retrieving the image of the product in the product view/page.
Note: above sample of codes were taken from Magento ver. 18.104.22.168