Adding a header image to a SharePoint list isn't a feature that's directly available in SharePoint lists, but you can work around it by customizing list with PowerApps, adding the header image through modern SharePoint page or adding the header image through list description.
Here are three situations:
1.For modern SharePoint lists, customizing list with PowerApps.
If you need more flexibility, you could customize the list with PowerApps to design a custom form that includes a header image at the top. PowerApps allows you to fully control the layout, adding any images, text, and other elements.
2.For modern SharePoint pages, use list web part and image web part.
Go to the modern SharePoint page -> Edit the page -> Click the "+" to add new web parts -> Add a list web part and an image web part.
Then choose the image web part -> Upload or link the image you want to use as the header.
3.For classic SharePoint lists, adding a header Image through list description.
Go to List Settings -> List name, description and navigation -> In the Description field, you can add HTML for an image, like <img src="URL_of_your_image" width="100%" />
.
Remember to replace "URL_of_your_image" with the actual URL of the image stored in your SharePoint library or another accessible location.
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