Hi @Tarek_Alber, welcome to the Asana Community Forum 
We do have an existing feature request for custom backgrounds. You can added your vote here: Custom Backgrounds
You can attach files or links in the Key Resources section of the Project Overview page:
Create a Project Overview to Showcase your Asana Project • Asana Product Guide
Finally, we offer the ability to add custom branding/company logo to your Asana instance with the Enterprise subscription.
I hope this helps 