In you first sheet/ screen shot I think none of the visualization is added so this will be the default view of any sheet.
Once you added at least one visualization the view will be just like 2nd screen shot.
You can set image to all of your sheets individually.
- Run the application.
- Once one sheet executed.
-Right side of the sheet you will able to see the image selection. Once none of the object is selected.
- you can upload the image or use default images.
Let me guess - the first one is pulished and the second is not?
The best way to take it under control is to use sheets screenshot as thumbnail (Changing the thumbnail of a sheet ‒ Qlik Sense). There's an extension to automate it, but I haven't tested it.