In Semplice, you can open your image full screen by clicking on it with the "Lightbox" feature. This feature is available in "Image" and "Gallery Grid" modules. While in the "Gallery Grid" module this feature enabled by default. In the "Image" module you will need to enable it by following the next steps:

Step 1. Add image

Drag and drop the "Image" module in the page editor and upload your image.

Step 2. Enable the "Lightbox" feature

In the "Image" settings "Module" tab you can find the "Links To" option, click on it, there you can enable the "Lightbox" by setting it to "Yes".

Step 3. Style the "Lightbox"

In "Customize" > "Advanced" you can find the "Lightbox" option, there you can change the opacity and color of the "Lightbox".