As explained in https://forum.affinity.serif.com/index.php?/topic/146713-link-to-image-layer-online-help-topic/, the US English help topic for "Image Layers" will not show up if a search is done on either "Image" or "Image Layers."
Works fine if the help is set to UK English.
Also, in both UK & US versions of the About Layers help topic there is no link back to the Image Layers help topic in the "Types of layer" section