Featured image for free blog

I'm using a free wordpress blog abdelhady.net, when I share (i.e on facebook) the home page or any post without a featured image, the post appears with a blank image, & this is the image I found in the header

<meta property="og:image" content="http://ift.tt/1uObZyU">

I don't know if this question is suitable here or not, but how could I choose a default image without using any feature from the premium package?