Deleted items are being backed up unnecessarily. So, go to your Photos, Recently Deleted album, and select all the items and delete them. When your next backup runs, these won't be included.
Now turn the phone off and back on. This will force the photo storage to be recalculated after your latest deletions.
Also, those same deleted photos are already backed up. So, get rid of your old backup... Go to settings, iCloud, storage, manage storage, iphone...
Delete your last backup at bottom of this page to allow your icloud storage to start over. (I suspect it never removes deleted items from your backup unless you do this.)
Turn your iphone backup back on. Go to Settings, icloud, backup to do that.
These steps will reduce your total storage when your next backup runs. Mine reduced from 735MB available to 2.6 GB available, freeing up almost 50%.