You may currently upload header images that have an aspect ratio of 16:9 or a dimension of 706 x 397 pixels with a maximum file size of 1 Megabyte (1024 Kilobytes). GIF, JPG and PNG are acceptable formats.
How do I resize an image?
There are many ways to resize an image. Most computers come with a default image editor. Besides these tools, there are many other software programs and online tools to crop and resize images.
Microsoft Photo Editor