Images cannot be shared between accounts. The only assets that can be 'seen' in a subaccount that exists in the master account are the email templates that have 'share with subaccounts' checked on the asset.
You can allow subaccounts to see email templates from the master account by:
- from the main left menu, go to Assets -> Email Templates -> select the email template and open with the 'pencil' icon
- select the 'gear'/Email Properties icon from the top of the right side menu
- check 'Template is visible to subaccounts?'
- click 'Save'