There are two images here, one "real" (the one which appears between the <a> and </a> tags), and the other which appears to be a spacer (notice it's got height and width attributes both set to 1). Assuming you want the full URL to the "real" image, your best bet would be to search for what's in between double quotes and after the sequence
src=
i.e. ereg("src=\"([\"]*)", $string, $image_url);
which means
match the sequence src=" then (zero or more characters that aren't "), and remember what's in the brackets.
$image_url[1] should hold what you're looking for. Now go back and read the ereg() page in the manual, it might make a little more sense =) [it's hard to figure out at the beginning!]
HTH,
AC