Replacing an image allows the user to replace and update a specific image file with the same name to the gallery. This is useful to swap out or update an image that is in multiple places across a website. 

To replace an image within the gallery, before uploading any new images to the gallery first select the tick box as shown below.

The user needs to make sure that the image wanting to be updated and the new image have the same name, file type and also that the site is published to see the changes on the live website.