I totally agree with you, the longer your conversations in Messages go on, the more photos, videos and other attachments get accumulated on your iPhone.
So, here how to delete messages attachments on iPhone:
- Open the Message app on your iPhone.
- Tap the conversation for which you want to delete attachments.
- Next scroll down to find photo, video or attachments that you want to delete.
- Now tap and hold the attachment and then tap on “More”.
- Next to the attachments you will see the circle checked marked. Now tap the trash can button to delete the attachments.
- Finally tap on “Delete Message” to delete the attachment.