How do i remove the white lines around my buttons?

So me and a friend started a project in school and we decided that i'll do most of the design part and he'll do most of the coding. I wanted to change the design on my buttons and changed the button image background and changed flatstyle to flat. Now there's this thin white line around every button instead. how can i get rid of it? Check attachment.

make flat appearance -> border size = 0 and check