why do the photos keep coming back after deleting from imessage?

Probably you have synced these Photos from your Mac/PC though iTunes. Un-sync then they be gone.

Un-Sync photos to your device

1. Connect your device to your Mac.
2. You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. To turn on Wi-Fi syncing, see Sync content between your Mac and iPhone or iPad over Wi-Fi.
3. In the Finder  on your Mac, select the device in the Finder sidebar.
4. 
5. Select Photos in the button bar.
6. 
7. Note: When iCloud Photos is turned on, no photo syncing options appear when you select Photos in the button bar.
8. Select “Sync Photos to your device from” tickbox, then choose Photos, Pictures or a folder from the pop-up menu.
9. 
10. Uncheck Sync photos
11. Click on Apply / Sync

I just spent a lot of time last week deleting photos and videos that took up a lot of storage space on my phone. General >Iphone Storage>Messages>. They are all back today and taking up the space again. I have Apple photo/icloud and use Google photo app as my primary photo app as I am not a Mac user. I need the space on my phone as I am about maxed out. I purchased additional space for my photos through my Google app so photo storage is not the issue. It is the videos and photos in Messenger. Any suggestions on how to quickly remove the photos and videos from the message storage and keep them off, would be greatly appreciated.