Google get image from url, which you posted. You can't post your own image as I know. To add image for your url you need to add image into page which you linked. Google supports opengraph and schema.org. Look at the answer from google here: Are there tags to specify the Google +1 story format in Google+ like og-meta for Facebook?
UPDATE:
I mean you can't set this image from your app. You must do it on your server. For example on cnn site you can find this code in html: <meta content="http://i2.cdn.turner.com/cnn/dam/assets/140512115559-pkg-nima-elbagir-abducted-girls-00014109-story-top.jpg" itemprop="thumbnailUrl" property="og:image">
on this page http://edition.cnn.com/2014/05/12/world/africa/nigeria-boko-haram-chibok-journey/index.html?hpt=hp_c2 When you share link on google plus (or Facebook), google go to your link and try to fetch this meta. If it found, google will add it to your post. How to describe it on your own website you can find on the link above.