I found a solution. Basicly you can do this:

$imageTag = new Zend_Form_Decorator_HtmlTag();
$imageTag->setOption('openOnly', true);
$imageTag->setTag('img');
$imageTag->setOption('src', 'buh.gif');
$imageTag->setOption('placement', Zend_Form_Decorator_Abstract::PREPEND);

and then use addDecorators(array($imageTag)); on whatever form element you want.