I don't know about using template in conditionals, but in the php file your editing/making you can do something like this:
PHP Code:
$url = "http://www.thesite.com/images/test.gif"
$check = fopen("$url", "rb")
if ($check)
{
//eval template when image does exist
}
else
{
//eval template for image does not exist
}
I think that should work...someone correct me if I'm wrong.