Images and other files are saved in file areas associated with the activity. Copying and pasting the image URLs from one activity to another is inviting disaster.
If you copy the URLs from the html editor
.. those URLs are draft file URLs, visible only to you, and disappear when Moodle purges temporary things. So too will your images.
If you copy the URLs from the display page of another activity
..changes in the original activity can break the newer one. And permissions can interfere too.
So to re-use images, you should
- select them from the recent files in the repository system .. Or
- upload them again .. Or
- If its the same type of activity, duplicate (or backup/restore) the original activity from the activity admin settings menu