To create a gallery inside one of your blog posts, follow these steps:
1. Open up one of your posts (Posts > All Posts) and click on the Add Media button above your main content area.
2. Click on the Create Gallery link and select or upload all of the images that you want to display in your post gallery.
3. Click on the blue Create a new galley button.
4. Now choose the number of columns for your gallery thumbnails and make sure “Link To” is set to Media File. Then click the Insert Gallery button.
Currently, my post gallery has 6 images in 3 columns, but the gallery thumbnail images are too small for my taste.
1. Go to Settings > Media and change the Thumbnail Size from the default 150px to something larger (or smaller)
2. Save changes and go to Plugins > Add New, search and install Regenerate Thumbnails plugin.
3. Go to Tools > Regen. Thumbnails and click on the “Regenerate All Thumbnails” button and wait till all your images are resized.
4. Refresh your blog post with the gallery and see how your new thumbnail size looks.