Deleting a Single Picture or Video from Messages in iOS
You can delete any single picture, video, GIF, or media file from the Messages app on iPhone, iPad, and iPod touch by using this trick. Keep in mind you can not undo this, so only delete media that you definitely want removed from the Messages app permanently:
Open the Messages app in iOS if you haven’t done so already, and go to the message conversation containing the picture or video you want to remove
Tap and hold on the picture (or video)
Choose “More” from the pop-up menu that shows up over the picture / video
Confirm the picture you want to delete is selected by looking at the checkmark, and then tap on the Trash icon in the corner
Confirm you want to delete the picture by choosing “Delete Message” – this does not delete the entire message it only deletes the picture / video that is selected as signified by the blue check mark
The picture or video is instantly removed and is no longer a part of the message thread, while the rest of the text portions of messages are maintained, and the pictures or videos that weren’t selected are also preserved.