Apply class on checkboxes collection

I would like to add a class on each instance of my collection. This is my checkboxes collection :

.cov-pick-row.w-row
      = f.input_field :inspiration_image_ids, 
        :collection => @inspiration_images.map {|img| [image_tag(img.image.url(:thumb)).html_safe, img.id] },
        class: 'image-cov-pick', 
       :include_blank => '(All)', 
       :multiple => true, 
       :selected => [''], as: :check_boxes

I would like an input like this :

            .cov-pick-row w-row
              .w-col.w-col-2 w-col-small-4.w-col-tiny-6
                .image-cov-pick
                  = image_tag('my_image1.jpg')
              .w-col.w-col-2 w-col-small-4.w-col-tiny-6
                .image-cov-pick
                  = image_tag('my_image2.jpg')
              ...

So my questions is : How to add this class .w-col.w-col-2 w-col-small-4.w-col-tiny-6 above the class image-cov-pick for each instances ?

Thank you for your help