"One of my users can see an image in the 'Promoted Items' section on the homepage. However, when he tries to view the asset details, he is being asked to log in again."

This is happening because that user does not have permission to view that asset.

You may wish to review this by either:

• Removing that image from the 'Promoted Items' section. This section should be used to promote items that all users can see.
• Changing the download permissions within Admin > Groups > Folder Permissions on that user's group, so that the user is allowed to see the asset.
• Adding the asset to a folder that the user's group is permitted to see.