How do i create a Pop-up Picture

I want to create a pop-up picture to appear as soon as visitors log on my website, how do i do this?

IMO, if its advertisement, i definately wouldnt do pop-ups. Either way, a
solution is below.
<script language="JavaScript" type="text/JavaScript">
window.open('the_page_to_open.html', 'Window_name', 'width=350, height=615,
location=no, menubar=no, status=no, toolbar=no, scrollbars=no, resizable=no')"
</script>
Place it in your header, and it should open automatically.

I am not 100% sure this will work.