Optimal format would be 16:9, max size 1281x720. But remember that in some cases only 360 of the height will be shown. Therefore, make sure the image does not have motives too close to the edges, especially on the upper and lower sides, as these may be cut out.
Since our platform has responsive design, this means we need to work with flexible photos, that can look good at various screens. Therefore, to obtain the best possible result, we advice you to use a photo with centered motive, no text, large margins to the sides, and a high quality. Try out how it looks if you are not sure - you can always change it.