To add more photos to your Publish gallery, put them in your post's source folder. Publish watches that folder, so new photos show in the gallery without you doing anything else.
How do I add more photos to my gallery?
To add more photos to your Publish gallery:
In Publish, click the folder button in the toolbar and click Show in Finder. Your source folder opens in Finder.
Open a new Finder window and find the photos you want to add.
Drag the photos into your source folder.
The new photos show in your gallery, ready to add to your post.
How do I add a folder of photos to my gallery?
To add a whole folder of photos to your Publish gallery:
In Publish, click the folder button in the toolbar and click Show in Finder.
Open a new Finder window and find the folder you want to add.
Drag the folder into your source folder.
Turn on Import images from subfolders: press ⌘, to open Preferences, go to the General tab and check Import images from subfolders. To learn more, see Can I include subfolders when adding images to a Publish post?
The photos from your new folder show in your gallery, ready to add to your post.