You have the ability to use a Custom Social Image. By default the preview image is the photo, gif or thumbnail of the video on the page. You can override this and upload your own JPEG that will be used as a preview image for everything. In order to do so, enable the feature.
Edit your Microsite template then click on the Sharing tab. Enable the Use Custom Social Image option. Finally, click within the box to Browse your file. You may also drag and drop your photo.
Social Card Settings Explained
Title - This will be used as the headline when shared to Facebook and other platforms that support rich objects.
Page Description - This description will be used when sharing the microsite to Facebook or other platforms that support rich objects.
Twitter Description - This is a Twitter only description that will be shared when viewed on their platform.