When you open an app on right side you'll find a Gear Symbol, click that to open the settings as seen in 2nd image.
Click on the Image box to browse through the available images to add your image instead.
You can save your company logo in below destination folder to add other than default qlik sense images.
You can put as many 'Text and Image' objects on your sheet as you want. If that is where you are talking about.
If you are asking how to add them to the sheet icon in the hub for this app then I am not sure how you can add more than one there without creating an image file outside of Qlik that contains them both.