When you send a link to a CRIBFLYER website in a text message (like on an iPhone) or post it on social media platforms like Facebook, the preview image is chosen using what's called an "Open Graph" image. This image is set automatically in your website's html source code, specifically the og:image meta tag.
CRIBFLYER uses the main property photo (1st position image) that you've uploaded on the Photos screen as the og:image (preview image). The 1st position image is the photo you have on the top left side of your gallery as shown in the screenshot below.
You can drag and drop photos to change their display order on the edit photos screen. Put the desired photo in the top left (1st position) to make it the main property photo and open graph preview image.
The main property photo is also the photo used by default in most of the Top Header section templates. Sometimes referring to as the "Hero Image" because it takes up most of the above the fold space when you first load the website.
Please note: Unfortunately, at this time, we don't have any way for you to specify a different photo other than the 1st position photo to be used as the open graph preview image (og:image).
When the property website's url is texted to an iPhone here's what the generated preview looks like.