Susanne_El
Did you try this post? It points to stack overflow for a solution.
https://community.qualtrics.com/XMcommunity/discussion/1960/code-to-lock-fix-an-image-question-to-the-top-of-a-page

You might be interested in freezeTopQ.

Hi there, you might also give the below CSS a try over in the Style section of the survey's Look & Feel:
.Skin #Questions {
    padding-top: 80px;
}

#LogoContainer {
  position: fixed;
  background: #fff !important;
  border-bottom: 1px solid #ddd;
  z-index: 9999;
}

Hello Tom_1842 ,
Thank you for your help. I have tried it out. The logo is fixed but as soon as I scroll the logo covers the questions. 

I think I see what you mean. Try using the below instead:
.Skin #Questions {
    padding-top: 80px;
}

#LogoContainer {
  top: 0;
  position: fixed;
  width: 100%;
  left: 0;
  right: 0;
  background: #fff !important;
  background-color: #fff;
  border-bottom: 1px solid #ddd;
  margin-top: 0;
  z-index: 999;
}
In the below, the survey is using Flat layout and the Logo is set to Style = Floating and Placement = Centered.