Simol
Sincere apologies. Somehow I missed your reply.
My wife had a similar problem when she inadvertently checked the iCloud Photo Library on her iPhone 7 and............. so many photos downloaded.
What I had to do was uncheck that setting. Don't be alarmed by the dire warning that the end of the universe is nigh if you do this.
Then the tedium really starts. After the unchecking, I had to manually remove all the photos.
Then, I discovered that I could have done this in a oner - always find out afterwards, eh!
Attach your iPhone via USB to your Mac - I'm presuming you have one - if not, it's the manual method.
Once attached, open Image Capture, found in Applications > Image Capture.
This will open a window, where on the left, you will see your iPhone. Click on this and all your photos will populate the screen to your right. Select all (Command + A), then "Delete". And they are removed from the iPhone.
BTW, if you have a Mac, it's best if you open the Photos app, go into its preferences > iCloud and check the iCloud Photo Library. This ensures that you have a copy of your entire library on the Mac as well as in the Cloud.
In fact, you might wish to copy your iPhone's pictures to the Mac if they're not already there, before the "uncheck and delete process" detailed above.
Ian