Good afternoon mikuru678,
Welcome to the Apple Support Communities!
I understand you would like to know how to delete photos from your iPhone, but still retrieve the deleted photos if needed from a new backup? Glad to see you posting this question before making these significant changes to your device.
To answer your question, I need to know if you have iCloud Photo Library enabled. (Open the Settings app on your iPhone, go to iCloud settings, and see if you have iCloud Photos (formerly called iCloud Photo Library) enabled (green button). If you do, you need to understand that these photos are not stored on your iPhone and won't be included in a backup because they are already in iCloud. Because this is a syncing service, anything you delete on your iPhone will automatically be deleted on all devices where iCloud is enabled with the same Apple ID.
If you do not have iCloud Photos enabled on your iPhone, and the photos are stored on your device in the Camera Roll instead of iCloud, then your photos will be included in the backup file when you back up using iCloud or iTunes on your computer. In this case, if you backup the iPhone, then delete the photos and create a second backup (preferably on consecutive days) then you can choose which backup to restore from (they won't be merged).
However, you can't access individual photos from a backup. So a better solution would be to import the photos that you "think" you want to delete to your computer in a separate folder (not in the Photos library) so that you can access the individual photos down the road if you change your mind. If you have a Mac computer, the Image Capture program would be good for this.
Again, if you have iCloud Photos enabled, on your Mac, you should export the photos that you don't think you want any longer to a folder, before deleting them from the iCloud Photo Library.
Then down the road, you can decide to permanently delete the folder, or import the images that you want back into your library.
I'll include some resources here to help you with your decision.
How to back up your iPhone, iPad, and iPod touch - I suggest you make an iCloud backup AND an iTunes backup
What does iCloud back up?
Transfer photos and videos from your iPhone, iPad, or iPod touch
Set up and use iCloud Photos
Delete photos on your iPhone, iPad, and iPod touch
mikuru678, I realize this is a lot of information, so if you can reply with a little more information (ie. are you using iCloud Photos or not, how many photos do you currently have on your iPhone, do you have a Mac or a PC, etc) I can adjust the response with more specific steps for your situation. I look forward to hearing back from you!
Enjoy your day!