Images output by Bricks Gallery have these classes by default (generally):
image bricks-lazy-load-isotope bricks-layout-inner css-filter
When using Perfmatters' Lazy Loading, Gallery images will collapse into each other.

Targeting the Gallery images using the default classes is not a good idea because they are generic.
This Pro tutorial shows how the bricks/element/settings filter can be used to add a custom class of bricks-gallery-image to fix the above problem.
After adding the PHP code and specifying our custom class (w/o the dot at the front) in Perfmatters' settings,

:

After implementing the tutorial, Gallery images will have these classes in the frontend:
image bricks-lazy-load-isotope bricks-layout-inner css-filter bricks-gallery-image
Add the following in child theme's functions.php (w/o the opening PHP tag) or a code snippets plugin:
This is a BricksLabs Pro tutorial.
For full access login below or create your BricksLabs Pro account