The patch is wrong because you are checking the theme EXT for the file:
This would be better but the image does not show up:
if(is_file(t3lib_extMgm::extPath($this->getExtensionName()) . 'Resources/Public/Images/screenshot.gif')) {
$this->previewImage      = t3lib_extMgm::extPath($this->getExtensionName()) . 'Resources/Public/Images/screenshot.gif';