Method 1: Double-clicking
1. Hold down the Command key and click on 2 images from the gallery.
2. While holding the Command key on the keyboard, double-click on one of the photos.

Method 2: Drag-and-Drop
1. Hold down the Command key and click on 2 images from the gallery.
2. While holding the Command key on the keyboard, drag-and-drop the photos to your desired spot on the post. A blue line will appear to show you wear they will be placed.

Try selecting more than 2 photos and place them side-by-side in your post.