Changing an app thumbnail isn't difficult, but it also isn't as simple as changing some of the other images on your page. Here is a quick rundown on how to change these images:
- On your page admin panel (at the top of your page if you are an admin), click on "Edit Page" and then "Update Info;"
- Select "Apps" in the right menu;
- Find the app you'd like to change the image of and click "edit settings;"
- Next to "Custom Tab Image," click on "change" (you can also change the app name here too);
- On the next screen, click "change;"
- Upload the new image and close the screen.
That's it. Your app thumbnail image is changed. Please note that you are not able to do this for Facebook's native apps like Photos and Notes.