The iPhone camera has quite high performance and high image quality. It's not unusual to say that because you can easily take a good-looking photo, the number of images has increased so that there is no free space in the internal storage if you notice. So, if you can retake it, you will need enough storage space if you don't feel like erasing the failed one.
However, if you delete photos to reclaim storage space, a single task is not enough. Unless deleted and deleted twice, storage space is not immediately restored.
This is because deleted photos go to "Recently Deleted Items". Photos taken with the camera app are stored in the area called library (camera roll) on the internal storage, but the first deletion only changes the display position on the app, so it is saved in the library as it is .
Deleted photos are kept in "Recently Deleted Items" for 30 days. Within 30 days after the transfer, you can always return to the state before deletion simply by performing the restoration process. Conversely, deleted photos will remain in your library for 30 days unless you do otherwise.
If you want the deleted photos to disappear immediately, open "Recently Deleted Items" in the album tab, select the target photos, and delete them. Deleting from "Recently Deleted Items" will result in a permanent deletion, and that will free up storage space. Of course, it's a permanent deletion, so you can't revive or undo the operation. Work carefully.