Rails Gem “Axlsx” / “Rails-Axlsx” - Adding Image

I try to add one image to my workbook/worksheet.

The Code :

    wb.add_worksheet(:name => "Doc1", :page_setup => setup, :print_options => options) do |sheet|

        img = File.expand_path('../logo.jpg', __FILE__)
        sheet.add_image(:image_src => img, :noMove => true) do |image|
            image.width = 7
            image.height = 6
            image.start_at 2, 2
        end

    ...

    end

But if i open the document there is no image on the worksheet, how can i fix this ?

And how is the correct path to the image? For this test i copy the jpg in my view folder, but usually all images in "app/assets/images/logo.jpg". I try it with img = File.expand_path('../assets/images/logo.jpg', __FILE__), but it fails with "No file found!"