You've got a typo in your code: where it should be
$('#item_two').hover(function(){
it says item_three instead.
That said, there's nothing wrong with using the jQuery removeClass function even if there's no classes, it won't fail. So you can do:
function imageInfo(){
$('#item_one').hover(function(){
$('#img_holder').removeClass().addClass('img_one');
$('#about_me_cont aside p').html('text explaining img 1');
});
$('#item_three').hover(function(){
$('#img_holder').removeClass().addClass('img_two');
$('#about_me_cont aside p').html('text explaining img 2');
});
$('#item_three').hover(function(){
$('#img_holder').removeClass().addClass('img_three');
$('#about_me_cont aside p').html('text explaining img 3');
});
}