When creating a custom image you will need to add SSH key access so you can clone the repository to your local machine to make changes.
SSH keys are not assigned to a specific image, but to your account. This means that anyone who has SSH access to custom images, has access to every custom image you have created under your account. You can easily add SSH key access when creating a custom image or when creating/updating a SSH key inside the servers module/servers/ssh-keys (this option will not appear until you have created your first custom image).
SSH keys cannot be used used in multiple accounts when they already have access to custom images.