On the gallery-grid page, you may notice that there are two fluid widgets.
The first fluid container is used to create the first row that consists of two featured gallery images. These two images use a separate fluid container in order to distinguish them from the rest of the images by making them bigger. The second fluid container is used to create the grid for the rest of your gallery images.
To Add Images to the Fluid Containers
Find the Fluid thumbnails at the upper left of the page. The column of thumbnails on the left is for the first fluid container. The column of thumbnails on the right is for the second fluid container.
Duplicate one of the thumbnails. Then, click on 'Fill'
Place your image by going down to 'Image' and by selecting your image file from your computer.
Additional Help With Fluid
More information on using Fluid can be found at the following links: