Adding custom fields/Gallery Field
From Joomla! Documentation
Gallery Field - Removed from Joomla 3.7
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Provides a drop down list of directories. All images in the selected directory will be shown in the frontend as gallery.
Special options within this field are:
The width of the thumbnails. The thumbnails will be resized on the fly.
The maximum width of the image. The image will be resized on the fly.
The maximum height of the image. The image will be resized on the fly.
Should the images from the subfolders also be included. Then you have to activated this option.
If activated it allows multiple values to be selected.
The filesystem path to the directory containing the image files to be listed. Root is pointing to images.
Creating the field
Let's say you create a field with the options shown in the next figure.
Using the field in the backend
In the backend while creating an article or a contact you see the field like in the following imageː
Using the field in the frontend
In the frontend, you can see the field as you see in the following image. The option Automatic display is responsible for the position of the field and your template is responsible for the design of the field.
Fields are only displayed in the frontend if you have filled them with data in the article. If it is not a required field, can you forget it?
Is not that great?