<form enctype="multipart/form-data" class="form-img" method="POST">
<div class="row" style="margin: 0px; font-size: 18px;">
    <div class="col-md-4">
        <img width="100%" height="auto" src="fotos/<?php echo $_SESSION['logo']; ?>"><br>
    </div>
    <div class="col-md-8">
        <label style="display: block;">Seleccione una imagen de perfil:</label>
        <div hidden><input name="image" type="file"></div>
        <div class="btn-only blue upload" style="margin: 0px;">Buscar</div>
        <span class="img-status"></span>
    </div>
</div><hr>
</form>

$(".upload").on("click", function(){
    $("[name=image]").trigger("click");
});
$("[name=image]").on("change", function(){
    $.ajax({
        url: "functions/cambiar_imagen.php",
        type: "post",
        data: $(".form-img").serialize(),
        success: function(e){
            alert(e);
        }
    });
});

<?php
session_start();
include("serverconfig.php");
include("codificar_clave.php");
include("log.php");

$target_path = "../fotos/";
$target_path = $target_path.basename($_FILES["image"]["name"]);
$temp_path = $_FILES["image"]["tmp_name"];

if(move_uploaded_file($temp_path, $target_path)){
    echo "El archivo ".basename($temp_path). " ha sido subido";
} else {
    echo "Ha ocurrido un error, trate de nuevo!";
}
?>