It looks like you're displaying your image via an AssetUrlHelpers. Your view code needs to be changed to the following:
<img src="<%= image_path(RED_GREEN_YELLOW_CLS[r.step_qties.last.ontime_indicator]) %>"
height="15" width="15" />
image_path
is going to properly add the assets
portion of the path, along with the hash generated by the asset pipeline, so that you get a correct url.