Add in a layer above base layer

If you want the logo to appear on multiple slides, consider putting it on the Slide Master. That's a great way to have something show on all slides, or on slides that use a specific master layout. Here's more info: Storyline 360: Using Slide Masters | Articulate - Community 

For the Slide Master, keep in mind that permanent objects (that is, shapes, pictures, and any object other than a placeholder) will always appear below objects put on the slide. And master objects can't be accessed for slide triggers. 

Another option if something needs to appear above base is to put it on a layer on the Slide Master. Add a trigger to the Slide Master that shows the layer when the timeline starts on the slide. Note that additional editing may be needed on slides that also use layers to ensure the master layer isn't hidden when a slide layer is shown.