These are the data in Mage_Page_Block_Html_Header
Object.
logo_src:
It represent image path for logo which defined from Admin in Logo Image Src
Field at System->Configuration->General->Design->Header
logo_alt:
It represent alt text for logo which defined from Admin in Logo Image Alt
Field at System->Configuration->General->Design->Header
Values are store in core_cofig_data
table with path value design/header/logo_src
and design/header/logo_alt
respectively ,so to change them pragmatically you can run such code
$store = Mage::app()->getStore();
$code = $store->getCode();
$config = Mage::getConfig();
$config->setNode("stores/$code/design/header/logo_src", 'value_to_set');