Hi misswinnielondon, thanks for bringing this up! The Pachyderm theme on WordPress.com doesn't support featured images — our theme team is updating the theme's tags to fix that.
However, as justjennifer mentioned, you should be able to get large images to appear for your blog posts in the Reader by using larger images in your posts. The reader looks for images that are at least 594px wide and have an aspect ratio above 0.8 (width/height).
@ justjennifer, currently, when a post with the Standard, Aside, Link, Quote, or Status post format contains a video or audio embed, that embed is pulled out and used in the Reader. Otherwise, it uses an image from the post.
If the post has the Gallery or Image post format, a video or audio embed in that post would not be used in the Reader - one or more images would be used instead. When the Reader looks for an image, it goes for the featured image first and then an image inserted into the post — if there are no images that meet the size criteria I mentioned above, it will display the image as a thumbnail instead of a large image.