When you compose a newsletter extracting automatically the latest posts from you blog, you have the option to extract a primary image to make the newsletter more visually appealing.
Sometime the image is not extracted correctly and there are a number of reasons not depending on the Newsletter plugin. Here a list and possible solutions.
Featured image not set
Newsletter, as first attempt, tries to extract the featured image from each post.
If you’re not using the featured image, Newsletter may fail to find something to show. You can start using the featured image or install a plugin which automatically set it (from pictures part of the post gallery).
This is the recommended solution since helps with other plugins as well. (try this search on wp.org)
No pictures “attached” to posts (custom galleries and like)
When you create a post you can upload one or more images which are “attached” to that post forming the “post media gallery”. If you’re managing the post medias using a third party solution, WordPress has no way to associate pictures to posts, hence Newsletter has no way to extract an image for the posts you’re embedding in a newsletter.
Again, you should check with your third party solution if it can interact with WordPress and provide a featured image when WordPress tries to extract it. This is usually possible since WordPress offers many ways to interact with the core functionalities.
I have pictures on my post content but Newsletter does not find them
Adding pictures to the post content does not means those pictures are “attached” to that post (see above). Newsletter does not try to analyse the post content to find pictures, just “asks” WordPress a list of attached pictures to get one to show.
Again, it is recommended to set the featured image on those posts, so there is an official picture to be used.
I would like to set the featured images, but my theme show it and I don’t want that
Ask the theme developer for an option to not show the featured image or a set of options to control when and how they should be displayed. Having a featured image connected to a post is of great value, you should not avoid it just a for small layout problem which can be fixed at the theme level.