Short: go to marketing material and right-click the file, or click on the ... button on the top right corner
Go to the Project management section and select "marketing material". All files shown in this section are visible in the showcase, by publishing them or unpublishing them you show or don't show them in the showcase
This action is always instant for everyone that has access to the showcase. This ensures that the information that is shared is always the information you want to be shared.