My iPhone has 25GB of iMessages - how can I archive for later retrieval

There just doesn't seem to be an easy way to back up iMessage - I have over 25GB of messages that contain memories and pictures synced across my Mac, iPhone and iPad via iCloud - and its very challenging to take out the pictures on my iPhone. So now I'm to the point where my iPhone's memory is maxed out and I'm having to delete messages one by one or save large attachments - its tedious on the phone. Why isn't there an easy way to use the big screen on the mac and back up with time machine??