Bookeo does not set the logo/image that goes with the Facebook post. Facebook takes the text and picture from the linked page. You can mark up your website with Open Graph tags to take control over how your content appears on Facebook, i.e. what image and text Facebook will use to decorate the post. For more information please see Facebook's tutorial here: https://developers.facebook.com/docs/sharing/webmasters
Please note these instructions are valid for any post/share/like that people make about your web site, and are not limited to Bookeo viral marketing. For this reason, whether you use viral marketing or not, it is usually a good idea to include Open Graph tags in all pages of your web site.
Image for the post
Where available, Facebook uses the meta description tags it finds in the page, or the og:image meta tag. So if you want to change the image used for the post, you should change the og:image meta tag in your web site, and Facebook will use that image. The picture is used as logo, and it may be stretches/resized if needed.
Text for the post
Where available, Facebook uses the meta description tag.
If no meta description tag is set for your page, Facebook will take the text it can find on your page.
You can try and add a meta tag with the description of your web site - see http://www.w3schools.com/tags/tag_meta.asp