The preferred dimensions required for an image in gallery is 675px x 310 px which gives an aspect ratio of nearly 2.2:1. If an image does not meet these dimensions then it may get stretched/squished and some area may be cropped to preserve the dimensions of an image. An image editor can be used to scale/crop/resize images.
There are two images gallery as of now : People and Science. The procedure for adding a new image to both of them is the same.