To create a Rollover Image Effect, follow these steps:

1. Add a gallery with the "Stacked" design

2. Add 2 images

3. Find a gallery block ID via the Chrome extension "Squarespace Collection/Block Identifier"

4. Copy the styles below to a text/code editor and replace #block with the gallery block ID that you found in the 3 step.

5. Add edited styles to Custom CSS

Note: You can skip 3-4 steps. Add this style to the page header code injection where is a gallery. Be careful, this code will affect all galleries with the "Stacked" design on a page.