The old version of Dynamics 365 Development toolkit was not supporting registration of images, unlike previous versions of Developer Toolkit.
However, in the new version of Developer Toolkit the support has been added.
In our CRM Explorer, once we have created a plugin, we can go Plug-In assemblies option in the navigation, select our plugin’s step and can add image.
This as expected updates the RegisterFile.crmregister file as well.
Hope it helps..