After you’ve removed the photos, keep in mind that any photos you’ve deleted in the last 30 days are still in your Photos app’s Recently Deleted folder, using up space, so make sure you delete them there as well. You may find Recently Deleted by going to the Albums tab and looking for it. Select Delete All from the upper right corner’s Select button.
You can also configure your iPhone to save smaller photos while storing the original, higher-resolution images on iCloud, which will save space. To enable this feature, go to Settings > Photos > Optimize iPhone Storage and click the box next to Optimize iPhone Storage.
Solution 2. Delete photos and videos that you have sent or received from others
Return to Settings, touch General, and then pick iPhone Storage if you don’t have time to determine what to remove or back up to iCloud and just want to start freeing up some space. When you get there, select Review Large Attachments, which is below the color-coded bar that indicates how much storage apps, photographs, the system, and other settings take up. You can erase photographs and videos you’ve sent or received in messages by placing your thumb on one, swiping to the left, and pressing the red Delete icon.
Prioritize getting rid of the first few of items at the top, which are likely to be videos, as they take up the most space.
