Make Archive Thumbnails Look Like Home's?

I need my Search, Archives, and Category pages to all have thumbnails just like the ones on the Home page.

I have combed through the code for over a day, but I barely know PHP, so I haven’t been able to find all the parts that influence the way these pages show up.

Does anyone know how to make the thumbnails on these pages look just like the ones on Home?

Thanks in advance!