Ingo2711 wrote: …
Are you using iCloud Photos … you can copy the photos from iCloud to your computer and delete them afterwards from your iPhone
Of course, if you're using iCloud Photos, you can view them full size and delete them at iCloud.com without ever downloading the pictures.
I think, without iCloud, the easiest thing is to do it on the phone. You can view them large and mark them--maybe with the Favorite symbol if you don't use that for other things, or by moving a bunch together and putting then in an album so you can trash the whole lot of them from the album at once. (Deleting the album itself won't erase the pictures--just the album.)
If you want to do it on your PC, then it might be worth it to buy a month's worth of iCloud Photos and do it that way-- $3 for 200 GB might be enough. You can cancel later.