If a topic is embedded into another topic, the content being embedded must be at least two characters in length.
An image counts as one character, so if you use an image in an embedded topic, the topic must contain at least one additional character.
The character length requirement for embedded topics enables the content boundaries to be in the correct location within the container topic.
If you neglect to follow this guideline, and your content contains only one character or object, the start and finish boundaries marking the embedded content will overlap and will not be interpreted correctly. In your container topic, the content will appear to be merged, or you will seem to lose some embedded content.
Best Practice for adding non-textual file objects such as images
When adding images to topics, rather than risk embedding a topic with a file object that does not contain any additional characters, best practice is to add the file object directly to the container content (not in a topic).
The file object is also single-sourced within the Library, so it functions like an embedded topic.