//To set custom image in Ribbon Header.
this.ribbonControlAdv1.RibbonHeaderImage = RibbonHeaderImage.Custom;
//To assign image in RibbonHeader
this.ribbonControlAdv1.CustomRibbonHeaderImage = imageListAdv1.Images;
We have also prepared a sample for your reference and it can be downloaded from the below location.
Sample Location : http://www.syncfusion.com/downloads/support/forum/120045/ze/RibbonCustomImage1589651438
Also, refer to the below screenshot for further reference.