Add ![ScreenShot](screenshot.png)
in the readme markdown as mentioned by many above. Replace screenshot.png with the name of the image you uploaded in your repository.
But here is a newbie tip when you upload the image (as I made this mistake myself):
ensure that your image name does not contain spaces. My original image was saved as "Screenshot day month year id.png". If you don't change the name to something like contentofimage.png, it won't appear as an image in your readme file.