We suggest the following: Aspect ratio = 5:2. Recommended size = 1200px x 480px to avoid image stretch or pixelation. The image is responsive, but we also suggest keeping important graphics within a ‘safe zone’ towards the middle / right of the cover graphic with a good sized border (approx 25px) around the edge (to help with display on different devices). If you are unfamiliar with ratios and image sizes, it can be tricky! This guidance is to help ease the pain, but it is not an exact science because there are so many different screen sizes, resolutions and viewing modes in the market. If you are concerned or need help, please contact support.