Pat Hawks

dirtysuds

Once activated, Gallery posts will have an excerpt that is a row of thumbnail images from that post, rather than some (often not descriptive) text. 
This excerpt should appear in your archive pages, and perhaps elsewhere, depending on your theme.
Note: The post must actually have the post type ‘Gallery’. It cannot just be a post with a gallery in it.

Gallery Excerpt

Replace the excerpt for posts with the post type 'Gallery' to a row of images from the post.

CAYdenberg

[catthumb id=”CategoryID” link=”true”] will find the most recent post in the specified Category with a featured image and return the thumbnail for that image. If link=”true” is specified, the image will link to the category specified.
You can optionally search custom post types with the <code>post_type</code> attribute, and specify the thumbnail size with the <code>size</code> attribute. IE <code>[catthumb id="CategoryID" link="true" post_type="restaurants" size="full"]</code>
The plugin will only search featured images, not attached images. It will not look for images embedded in the post itself, or other attached images. If a category does not have any posts with featured images, this plugin will not return a thumbnail image, even if the category has posts with attached images.

Category Thumbnail

Adds shortcode [catthumb] to embed a thumbnail image for a category.

Gallery Excerpt vs Category Thumbnail