You don't need to go into the QMC.
Simply do it in the app itself.
Open the App information by clicking on the 3dots next to the app name at the top.
Then click on the edit icon.
Then click on the icon in the middle of the image.
Then choose an image from the default library or upload a new image.
Save the changes.
Thank you for your answer but i'm not looking to change a thumbnail picture in particular, i want to change the default picture of every created App, the second you create it. The default picture is the one with the 3 rings in orange and green but i want to use a different one than this.