seeElement() may fit your need, for instance:
$I->seeElement('//img[@src="/pixel.jpg"]');
please do see doc
and, in order to make test more readable, I'd recommend to wrap the actual call to seeElement() in helper and in test have something like this
$I->seeImageWithSource('/pixel.jpg')
public function seeImageWithSource($image_url)
{
$phpBrowser = $this->getModule('PhpBrowser');
$phpBrowser->seeElement('//img[@src="'.$image_url.'"]');
}