Considering these are mostly just duplicates you already have on your photo roll, it's needless space being used.
For phones with iOS 11, go to Settings > General > iPhone Storage > Review Large Attachments. This will bring up all the attachments you have sent, and by pressing edit you can easily select the photos and videos you wish to delete.
For iPhones running previous iOS, the only option is go through each individual conversation and long-press on a photo or video. Tap "More" on the pop-up and you can select the photos and videos you want to delete.
Debbie Mayo-Smith is one of New Zealand's most sought after motivational speakers and trainers.. Sign up for her monthly quick tip newsletter here. http://debbiespeaks.com/newsletter/newsletter-sign-up/ To have Debbie speak at your next event or train your team call 64 27 575 5359 or visit her www.debbiespeaks.com