For search results, add "image" to catalogsearch.xml:
<block type="catalog/product_list" name="search_result_list" template="catalog/product/list.phtml">
<action method="addAttribute"><attribute>image</attribute></action></block>
For product listing, add "image" to catalog.xml:
<block type="catalog/product_list" name="product_list" template="catalog/product/list.phtml">
<action method="addAttribute"><attribute>image</attribute></action></block>
Reference image in catalog >> product >> list.phtml:
<img src="<?php echo $this->helper('catalog/image')->init($_product, 'image')->resize(200,null); ?>" width="200" height="200" alt="<?php echo $this->stripTags($this->getImageLabel($_product, 'image'), null, true) ?>" />