Begin by signing in and going to your Dashboard. Open the post where you want to upload a Featured Image. Look toward the bottom right corner – below Categories and Tags – and you’ll see the spot for the Featured Image.
Find a featured image in your Library. If it is vertical, like the photo above, you’ll need to crop it. I suggest that the photo have proportions of roughly 2×1, that is, a photo whose width is about double its height. Note: If you’d like to be even more exacting, I believe that the window size that will display the photo measures 1,024 x 540. If you upload a photo that is 1,600 pixels wide, which should give you acceptable resolution, then the photo size would be 1,600 x 846.