You can set a default or fallback image to be used on Posts modules, for instance a posts module displayed on your homepage or News page. The system will display the fallback image if you have not chosen a featured image at the post level. 

To set or change the fallback featured image open the page containing the module in the page builder, then open the Posts module settings (might also be call Content Grid or similar). Then remove the current featured image, and then choose your replacement image, save, and publish the page.