The Photo app and iCloud sync with each other - so you can't delete it from the Photo app without removing it from iCloud.
If it is to save on space, then you can check whether Optimize iPhone Storage is activated.
The feature is enabled by default, but check anyway. Go to:
Settings > Apple ID > iCloud > Photos.
Alternatively. Then you can move your photos to iCloud Drive. That way, your photos stay in iCloud, but you can offload them from your iPhone's memory.
1. Create a folder in the Files app (in the iCloud Drive section)
2. Open the Photo app, select the photos you want to move
3. Tap the Share button (square with up arrow) and select "Save to Files"
4. Find the folder you created and save there.
Go to the folder in iCloud Drive. Find your photos and mark them. (Remember that it may take some time before the images are uploaded, it depends on how many you upload and your connection). Tap the small round icon with 3 dots in the lower right corner and select "Remove Downloads" - this will offload the images and you'll see a cloud with an arrow pointing down from each image.
Your photos are still in iCloud, but do not take up space on your iPhone, and you will always be able to see them from your iPhone.
Now you can actually also delete the pictures from the Photo app, but BEFORE YOU DO THAT - check https://icloud.com > Files that your pictures are there. It would be a shame to delete the photos from the Photo app, only to find out that the photos in iCloud Drive were not uploaded.
