As a new feature, when you upload an image to a post you have the choice of how large it should display. You can pick the size by looking in your preview view and clicking the ‘50% / 75% / 100%’ links. The 100% is the default. Here you can see I picked the ‘50%’ option:
This allows us to make mini galleries if we want. When you click an image it goes to the large display and you can then use the ‘next / previous’ white arrows on the left and right to cycle between all images in the post.
To make images ‘squish up’ you might need to remove some spaces between them, but the preview view is useful for making sure you got this right.