To support Retina or HiDPI displays, the icons provided by the XMLNodeRendererCustomizer should be backed up by a copy of larger size using the proper Retina/HiDPI naming convention.
For example, for the title element, if the XMLNodeRendererCustomizer returns the path ${framework}/images/myImg.png, then in order to support Retina images with a scaling factor of 2, an extra file (myImg@2x.png)should be added to the same images directory (${framework}/images/myImg@2x.png). If the higher resolution icon (the @2x file) does not exist, the normal icon is scaled and used instead.
For more information about using Retina/HiDPI images, refer to the Using Retina/HiDPI Images in Author Mode section.