Facebook itself decides, based on internal criteria, which content (text and images) is shown in the preview of a shared link. By default, the top most image on your website (which is at least 1200 * 630 pixels in size) is used.
Help Facebook decide
You can suggest to Facebook which picture should be used as standard in the preview when sharing your Jimdo Creator website. Facebook itself has an extensive guide on this. You can add the tags Facebook describes in the Edit Head section of your website by going in the menu to Settings > Edit Head.
Finding the image address
To find the path to the image, look at your website in the View Mode. Find the image you want to use and right click on it to view the options. You can copy the path by choosing Copy image address and then inserting this address, along with the rest of the code, in to the Edit Head section.