Please make sure to use the images of the following sizes (in px).

Logo
width: 187px;
height: 44px;

Header ad image
width: 728px;
height: 90px;

Banner large image
width: 960px;
height: 539px;

Banner small image
width: 480px;
height: 270px;

Home page top news large image
width: 555px;
height: 494px;

Home page top news small image
width: 265px;
height: 186px;

Home page slider section
width: 480px;
height: 363px;

Blog post image ( with sidebar) 
width: 750px;
height: 410px;

Full width Post
width: 1170px;
height: 410px;

Widget recent post
width: 81px;
height: 76px;

Widget popular post
width: 292px;
height: 195px;

Author
width: 105px;
height: 105px;