You see the problem – the image you chose is narrow so, on a computer screen, it has a nasty grey background. This code is tested with Twenty Fourteen. You need premium or business to make it permanent – but you can try it using your CUSTOMIZE option on a free blog.

body.blog .content-area {
padding-top: 0px;
}
.post-thumbnail {
background: none;
text-align: center;
}