According to the shared image gallery docs linked by Hannel:
Q. How can I list all the Shared Image Gallery resources across subscriptions?
A. In order to list all the Shared Image Gallery resources across
subscriptions that you have access to on the Azure portal, follow the
Open the Azure portal. Go to All Resources. Select all the
subscriptions under which you’d like to list all the resources. Look
for resources of type Private gallery.
To see the image definitions and image versions, you should also
select Show hidden types.
To list all the Shared Image Gallery resources across subscriptions
that you have permissions to, use the following command in the Azure
az account list -otsv --query ".id" | xargs -n 1 az sig list --subscription
Q. How do I share my images across subscriptions?
A. You can share images across subscriptions using Role Based Access
Control (RBAC). Any user that has read permissions to an image
version, even across subscriptions, will be able to deploy a Virtual
Machine using the image version.
Q. Can I move my existing image to the shared image gallery?
A. Yes. There are 3 scenarios based on the types of images you may
Scenario 1: If you have a managed image, then you can create an image
definition and image version from it.
Scenario 2: If you have an unmanaged generalized image, you can create
a managed image from it, and then create an image definition and image
version from it.
Scenario 3: If you have a VHD in your local file system, then you need
to upload the VHD, create a managed image, then you can create and
image definition and image version from it.
If the VHD is of a Windows VM, see Upload a generalized VHD. If the
VHD is for a Linux VM, see Upload a VHD
Q. Can I create a shared image gallery, image definition, and image version through the Azure portal?
A. No, currently we do not support the creation of any of the Shared
Image Gallery resources through Azure portal. However, we do support
the creation of the Shared Image Gallery resources through CLI,
Templates, and SDKs. PowerShell will also be released soon.