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.

When adding images, a description of the image or a meaningful file name should be used so that the image can be searched for in the Media Library

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.