Can you tell us which template specifically? One of the free ones available under the New window dialog?
Usually, it's just a case of right-clicking the title of the placeholder layer in question > Replace Contents. (Use Ctrl (Cmd on Mac) + T to resize the image if it's too big.)
Thank you very much for your answer.
I use the free template ‘PhotoCollage Layouts’.
I have succeeded to replace contents In the template. I will see later to resize with T.
A documentation would be useful to explain how to get the result from the template with replaced contents.
Most of the time it's just replacing Smart Objects with your own assets/content, anyway.
Replacing the contents of a Smart Object (Link takes you to a free chapter in the PS CC 2017 course at lynda.com. Even though it's the previous version, the rules still apply.)