Laravel Image Upload using ajax 500 Internal Server Error

I am trying to upload image using ajax in laravel 5.2.but still i am getting error 500 Internal Server Error in route. when i am trying to upload image using ajax request the browser shown correct route path but still i am not getting reason why it still showing error to me.

HTML

<!-- CHANGE AVATAR TAB -->
                        <div class="tab-pane" id="tab_1_2">

                            <div class="uploadimagediv"></div> 
                                

                                <div class="form-group">
                                    <div class="fileinput fileinput-new" data-provides="fileinput">
                                        <div class="fileinput-new thumbnail" style="width: 200px; height: 150px;">
                                            <img src="http://www.placehold.it/200x150/EFEFEF/AAAAAA&amp;text=no+image" alt=""/>
                                        </div>
                                        <div class="fileinput-preview fileinput-exists thumbnail" style="max-width: 200px; max-height: 150px;">
                                        </div>
                                        <div>
                                            <span class="btn default btn-file">
                                            <span class="fileinput-new">
                                            Select image </span>
                                            <span class="fileinput-exists">
                                            Change </span>
                                            <p class="text-danger" id="error_image"></p>
                                            <input type="file" id="picture" name="picture"/>
                                            --}}
                                            </span>
                                            <span class="error alert-danger"></span>
                                            <a href="javascript:;" class="btn default fileinput-exists" data-dismiss="fileinput">
                                            Remove </a>
                                        </div>
                                    </div>
                                    <div class="clearfix margin-top-10">

                                    </div>
                                </div>
                                <div class="margin-top-10">
                                    
                                    

                                    --}}
                                    <a href="javascript:;" class="btn default">
                                    Cancel </a>
                                </div>
                            
                        </div>
                        <!-- END CHANGE AVATAR TAB -->

Route

Route::group(['prefix' => 'admin'], function ()
{
    Route::controller('/','DashboardController');
});

Ajax

$(document).on('click', '#updatepicture', function($e)
{
    $e.preventDefault();
    // send an ajax request
    $("#error_image").empty();
    $.ajax(
    {

        url: 'avatar',
        processData: false,
        contentType: false,
        type: "post",//use for update
        data: new FormData($("#updateprofileimage")[0]),
        success: function(data)
        {
            if(data.status)
            {           
                $("#uploadimagediv").html('<div class="alert alert-success"><button type="button" class="close">×</button>'+data.message+'</div>');
                window.setTimeout(function()
                {
                    $(".alert").fadeTo(500, 0).slideUp(500, function()
                    {
                        $(this).remove(); 
                    });
                }, 5000);
                $('.alert .close').on("click", function(e)
                {
                    $(this).parent().fadeTo(500, 0).slideUp(500);
                });
                //console.log(data);
                //$("#updateprofileimage")[0].reset();
                //window.location.href = "http://localhost/laravel/admin/profile";
            }
            else
            {
                errors = data.errors;

                for(error in errors)
                {
                    $("#error_"+error.title).html(error.message);
                }
            }
        },
        error: function(xhr)
        {
            if(xhr.status == 422)
            {
                errors = xhr.responseJSON;
                for(error in errors)
                {
                    $("#error_"+error).html(errors[error][0]);
                }
            }
        }
    });
});

Error is :"NetworkError: 500 Internal Server Error - http://localhost/laravel/admin/avatar"

please suggest me where i am getting wrong.

Controller is

public function postAvatar(ImageUploadRequest $request)
{
  ---
}

via ashish bansal