How do I customize my asset's network node icon?

To change the network node icon, navigate to Type Properties > Basic > Icon and browse to the desired file. This will also change the icon shown in the tab menu.
e.g.: opdef:/Object/assetname?icon.pic

To change the help card icon, navigate to Type Properties > Help and add a path to the image using to following syntax:
#largeicon: opdef:/Object/assetname?image.png
  Note: the help card does not support .pic images

To embed images in the help card, first add the files via the Extras tab and use the following syntax in Type Properties > Help:
[Image:opdef:/Object/assetname?image.png]

Since icons are cached on startup, you may need to restart Houdini to see your changes.

Namespaces:

When your asset name has a namespace such as "Orbolt::assetname", the namespace must come before the object type:

opdef:/Orbolt::Object/assetname?image.png