To display image in custom admin grid you need to add a column in grid.php with following code:

    $this->addColumn('Database_Field_Name', array(
              'header'    => Mage::helper('ModuleName')->__('Image'),
              'align'     =>'left',
              'type' 	  => 'image',
              'index'     => 'Database_Field_Name',
              'renderer' => 'modulename/adminhtml_modulename_renderer_image', //get the image HTML code class name in render class should be written in small caps
    		  'filter'    => false,
              'sortable'  => false,
    ));

After that you need to create following image renderer file on module's `Block/Adminhtml/ModuleName/Renderer/Image.php` location

    <?php
    class NameSpace_ModuleName_Block_Adminhtml_ModuleName_Renderer_Image extends Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Abstract{
       public function render(Varien_Object $row)   {
    		$html = '<img id="' . $this->getColumn()->getId() . '" src="'.Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_MEDIA).$row->getData($this->getColumn()->getIndex()) . '"';
    		$html .= '/>';
    		return $html;
    	}
    }
    ?>