[TYPO3] EXT develop path question

[Rocky Ou]

Hey list,

I need my Front end plugin render some image from EXT's res directory. I 
use the following code does not give me the image in the browser. BTW, I 
tried extPath as well.

	$resPath=''.t3lib_extMgm::extRelPath($this->extKey).'';
	$content.='<h5><img src="'.$resPath.'res/header.jpg"></h5>

but the following code gives me the image.
            $content.='<h5><img 
src="http://localhost/~lover/dummy40/typo3conf/ext/da_c_tet/res/header.jpg"></img></h5>';

Can any of you give me a hint on why this happens please?

Thanks in advance!
Rocky