[iMore.com]

Pictures and videos stored in the Messages app can take up lots of storage space on your iPhone and iPad.
If you've already saved a lot of them, there's little reason to keep them hanging around in the Messages app. Deleting them all one by one would be a daunting and tedious task. Instead, you can delete multiple images or videos at once in the Messages app. You've just got to know where to look!

Full story from the iMore Blog...