regular expression help, validate url and extension

hi,

i have an expression that validates URLs but i need it modified so that it only allows certain file extenstions, or if you have another fresh example that will do fine

i need an expression that will only allow a url with a file name with one of the following extensions, jpg, jpeg, gif, png

for example

http://domain.com/picture.jpg - valid
http://domain.com/picture.php - invalid

http://domain.co.uk/folder/picture.jpg - valid
http://domain.co.uk/folder/ - invalid

etc etc

here is the function i currently have:

function valiurl($url) {
 if (eregi("^((ht|f)tp://)((([a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3}))|(([0-9]{1,3}\.){3}([0-9]{1,3})))((/|\?)[a-z0-9~#%&'_\+=:\?\.-]*)*)$", $url)) {
  return true;
 }
 else {
  return false;
 }
}

any help will be much appreciated, thank you