At what point are you calling the function ? It should work like that:
function showHint(str, dir) {
$("#wrapper").html( '<span id="loader"></span>');
$('#wrapper').load(str +'.php?' + $.param(dir), function() {
$('.file').preimage();
});
}
$(document).ready(function() {
showHint('some-url', 'some-param');
});