You can share your selected custom image with your social buttons. Follow the steps to make it:
1. Create your social buttons (How to create Social Buttons?)
2. Go to the page you wish to have the buttons
3. Click "Insert Social Media Buttons"
4. Select the Social Media Button you wish
5. Select the image you want to share, and "Insert" it.
You can have this using the shortcode of your buttons (add your buttons' id number and the image URL inside) :
[sgmb id="5" customimageurl="https://yoursite.com/image.jpg" ]
This is the result on the page, when you share it (the FB in this example)