In ACC if you would like to use your own brand logo in an out of the box theme of a web app they can update it according to the documentation https://experienceleague.adobe.com/docs/campaign-classic/using/designing-content/web-forms/publishing-a-web-form.html?lang=en
They need to go to the Properties of the web app in question, Rendering - click on magnifying glass icon next to the chosen theme - Page layout - change the path in the Image field.
The images are available under Administration - Configuration - Images. The customer’s logo needs to be added there.
These images are placed in the back end directory of the instance datakit\nms\fra\img\activities or datakit\nms\eng\img\activities (eng or fra, depending on the language of the instance).
To have a new image available in this directory (and in Images), a JIRA ticket needs to be raised to the TechOps team, which has admin rights on the backend directories.
For on-premise instances, the customer can add images to the datakit themselves.
The uploaded image doesn’t have to be visible from the Campaign Client. The correct path is enough to use as a new logo.