The recommended images sizes are

With Sidebar: 832 x 475

Without Sidebar: 1140 x 475

Layout Post: 246 x 246

Banner: 1920 x 910

About Page: 520 x 520

Events: 185 x 185

Testimonials: 570 x 474

Team Members: 360 x 385

Popular Post: 60 x 60

Archive: 360 x 268

Gallery: 270 x 270