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What is a “central model”?

Are you talking about your logo? If yes, it does exactly what it's supposed to do, in cell phone or tablet mode it becomes smaller and is in the middle...
Maybe you can show your problem with screenshots
which browser and system do you use?
Which template do you use?

Do you know that you can also set an extra logo for responsive views in the template settings, in your template style under: "Logo"? There you can also specify if the logo should be used on which screen size...

Hi, I'm not talking about the logo, the logo is fine.
The problem is with the header image. This image doesn't fit if I look at the site with my phone: in fact from the PC I can see the image well, on the phone I only see a black part.
The model I used is "J51 - Central"

Thanks Elisa

Unfortunately, I can not help you there.
Have you tried another image?
Ciaran may be able to provide a template update from the developer. But I don't know the settings or your Template Central...
In the example on the Joomla51.com site the same thing happens. It doesn't seem responsive... Probably due to the template...
Are you using Joomla 4 or 3?
Let's hope that ciaran gets in touch here...

Hi Elisa

May I suggest sending temporary administration access to your Joomla installation to info@joomla51.com and we will examine this issue further. For our reference please paste a link to this post in your email.

Ciaran

Thank you Elisa

Admittedly the header area is designed to be fluid as you request, instead design to have a fixed height where the image is set to always cover the space available. Regardless I have amended the CSS fields in your templates Rresponsive Options tab to adjust the height for each screen size which will hopefully bring you closer to the desired result.

Ciaran

Thank you so much! I'll try to find another suitable image, but it's already better.
Thanks again
Elisa