Images should be as large as possible (but not larger than 2 MB, which is the limit of our site) to prevent blurry displays and should have a landscape orientation of approximately the correct ratio to prevent unwanted cropping.
Some details from the MH Magazine website:
In general it’s recommended to always upload images with size of at least 1030 x 438px – this will ensure that WordPress can generate all thumbnails:
1030 x 438px (Thumbnails for the MH Slider widget)
678 x 381px (Thumbnails for featured images on posts, archives and widgets)
678 x 509px (Thumbnails for some custom widgets)
326 x 245px (Thumbnails for archives and widgets)
80 x 60px (Thumbnails for some custom widgets)
Additional information here.