Hello,
Thank you for contacting us.
Google usually picks up the first image it finds on the page that is suitable to show up on SERPs. Having said that, this is a completely automated process and no SEO plugin or tool has control over that.
Best practices to get thumbnails are to add images on your page that you want to display on SERPs and also include a featured image, ensure that the images contain proper ALT texts. This will give Google the correct indications and it might show the picture in the mobile search results.
Hope that helps.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.