Make a duplicate of the app. Then Edit it and click on the properties for the app.
Click on the icon and see what is displayed.
On my version there are a bunch of images that are In app. If you see them then chose the one for the app that you want.
Republish that app.
I am guessing that those image files are probably not there and that is why the image is broken.
Mine used to do this all the time. You can click on the app and then the Edit button in the app header. Then, click the image for the app which will bring up the media library. You should see a list of .png files and one of them should end with "_Thumbnail.png" which you can select and click Insert. If you don't see the thumbnail file you can either upload a new one (which I have attached) or I have had success just running the Repair on my Qlik Sense Installation.