Hello @andybaines,
thanks for reaching out! From what you are describing I am most certain that it must be related to missing open graph tags in your posts META data. When sending out link tweets to Twitter the network automatically scrolls the META data of a webpage and generates the link preview with title, excerpt and image itself. In order for Twitter to be able to do this, its needs the respective tags e.g op:images or og:description.
Whether open graph tags are automatically set can vary from theme to theme. You copuld either set them yourself by editing your HTML or simply apply another plugin which will do the job for you. We recommend either using Yoast SEO or OG.
Since our last update, you can choose between two diffferent post formats for Twitter. Either the link format or the image format. If you choose the link format and have set open graph tags your tweet will automatically show your posts featured image. If you choose the image format (either in your B2S settings or right there in your posts preview) you are able to select a specific image.
Just for making sure your issue is indeed related to missing open graph tags, please try to send out an image tweet and see whether the image is displayed. If not, you might also want to check your .htaccess file and make sure you have not activated any hot link protection.
You are also welcome to contact us via e-mail at customer-service at blog2social dot com.
Best regards!