Hello, I would like to know how I can display the image "under construction or under construction" for visitors? I know it's with an html file but I don't know how to do it.

Could you help me ?

I thank you in advance for your help and wish you a good day.

Cordially.

Are you wanting visitors to see the under construction while you're able to work on your forum itself setting everything up?  If so, all you need to do is turn the board offline.  

To do this, goto: ACP > System > Settings > General Configuration

UNCHECK the option for "Site Online".  When you uncheck it, a box will appear to let you control what is shown to non-admins.  Save your change and presto!

To test this, pull up a different browser not currently logged in.  It should show the offline message you've configured.  Only if you're logged in as an Admin will you see your work in progress!

precisely I don't want visitors to see that I'm editing the site, I would like them to see the image that is attached

That’s a perfect use case for the Site Online feature.   When you access the editor to enter your message, you can add images and such just like making a forum post. 

When you’re ready for the public to see your work, just go back to the same area and turn the board back on. 

ok i didn't know it would work like that i think it needed an html file

Nope. You don’t need it when using IPB. You might if you were using something else!