To add a background image to your Azure AD app without a subscription, you can follow these steps:
Sign in to the Azure portal using your Azure AD admin credentials. Navigate to the Azure AD app that you want to add a background image to. Click on the "Branding" option in the left menu. Under the "Background" section, click on the "Add" button. Choose the image file that you want to use as your background image and upload it. After uploading the image, you can preview it by clicking on the "Preview" button. Once you are satisfied with the background image, click on the "Save" button to save the changes.
Note: Azure AD Free does not support custom branding, so the background image option may not be available for all types of Azure AD apps. If you do not see the "Background" option under the "Branding" section, it may not be supported for your app.