Looks like you are wanting to change the thumbnail of your app:
Changing the thumbnail of an app
You can replace the default thumbnail of an app with another thumbnail, to make it easier to distinguish between apps in the hub. You can use one of the default images, or an image of your own.
Do the following:
- In the app overview, click @ in the app details area.
- Click 0 on the default thumbnail.The Media library opens.
- Click on a folder in the media library, for example In app or Default.
- Select the image you want to use as a thumbnail for the app and click Insert.
- Click m to stop editing.
The image you selected is now used as a thumbnail for the app.
The following formats are supported: .png, .jpg, .jpeg, and .gif.
For Qlik Sense: You can upload images to the In app folder in the media library. You need to use the Qlik Management Console to upload images to the default folder.
For Qlik Sense Desktop: If the default folder is empty, or you want to add your own images, you find the folder at: <user>\Documents\Qlik\Sense\Content\Default. When moving an app between installations, images are bundled and saved in the qvf file together with the rest of the contents of the app. You find the bundled images in the In app folder in the media library.
The default location is \My Documents\Qlik\Sense\Content\Default, may be they have been deleted,Here's the ones from my install, copy them into the folder and see what happens.