Step 1
Create a WordPress Gallery. This will become the Photo Set.

Step 2
Switch the editor into Text view.

Step 3
Find the [gallery] and change it to [photoset]

Step 4
After the “IDs”, add a parameter for “layout”. E.g: [photoset ids="100, 110, 152" layout="2, 1"]

Step 5
The numbers in “Layout” represent the number of photos you want in each row. (i.e. You’d like the first row to have 3 photos, the next to have 1 photo and the last row to have 2 photos, you’d type 3,1,2). The layout number must add up to the total number of photos referenced, otherwise the photoset will display incorrectly.

Additionally

Photosets will show a caption on rollover. Fill out a caption for each photo now, or while building the gallery. If you need to edit an existing photoset’s captions follow these steps.

Edit Existing Photoset Captions Tutorial