In Messages for iOS, can I delete an individual photos from a conversation?
Go into the conversation on your iOS device (assuming iOS10). In the top right corner you should see an information symbol ("i" in a circle). Click on it. Then scroll down to the picture you want to delete. Then tap and hold the picture. Three options will come up: copy, delete, more. Select delete. If you're using an iPhone 6s or newer, be careful not to force touch the image instead of tapping and holding.