Go to app/design/frontend/{your package}/{your theme}/template/checkout/onepage/payment/methods.phtml if you haven't it in your theme, copy it from the {your package}/default or base/default
add this: <img src="<?php echo $this->getSkinUrl('images/'.$_code.'.jpg') ?>" alt="<?php echo $this->escapeHtml($this->getMethodTitle($_method)) ?>" /> after p_method_<?php echo $_code ?>
So you'll have something like this:
<label for="p_method_<?php echo $_code ?>"><img src="<?php echo $this->getSkinUrl('images/'.$_code.'.png') ?>" alt="<?php echo $this->escapeHtml($this->getMethodTitle($_method)) ?>" /> <?php echo $this->escapeHtml($this->getMethodTitle($_method)) ?> <?php echo $this->getMethodLabelAfterHtml($_method) ?></label>