How to transfer photos from computer to iPhone?

Sync your content using Wi-Fi

1. Connect your device to your computer with a USB cable, then open iTunes and select your device.
2. Click Summary on the left side of the iTunes window.
3. Select "Sync with this [device] over Wi-Fi."
4. Click Apply.

Use iTunes to sync your iPhone, iPad, or iPod with your ...

WARNING: If you delete an automatically synced item from your PC / Mac, the deleted item is removed from your device the next time you sync.

What you can sync with iTunes

• Albums, songs, playlists, movies, TV shows, podcasts, and audiobooks.
• Photos and videos.
• Contacts and calendars.

Sync or remove content using iTunes

1. Open iTunes and connect your device to your computer with a USB cable.
2. Click the device icon in the upper-left corner of the iTunes window. Learn what to do if you don't see the icon. 
3. 
4. From the list under Settings on the left side of the iTunes window, click the type of content that you want to sync or remove.* To turn syncing on for a content type, select the checkbox next to Sync.
5. 
6. Select the checkbox next to each item that you want to sync.
7. Click the Apply button in the lower-right corner of the window. If syncing doesn't start automatically, click the Sync button.

If you are looking for the photos the same way as they were on your iPhone prior to deletion then the only way is to restore from a backup from iCloud or iTunes. If you have a backup --> Restore your iPhone, iPad, or iPod touch from a backup ...

I'm afraid you can't. On both Mac and PC, if you transfer a photo from your iPhone, both of its "Modified" and "Created" time stamp would be set to the exact moment it is transferred, so you won't be able to restore your transferred photos back to its original place in your iPhone Photo Gallery.

You can check the "Created" and "Modified" time stamp of your photos. On a Mac, right click(double finger click) a photo, choose 'Get Info'. On a PC, right click a photo and choose 'Properties'.

Exactly the same -- you can't, just iTunes backup restore that has these photos with all consequences. Best you can do is iTunes sync, but really I don't know how it handles metadata. If you want to put them only in Camera roll, test Google Photos with paid plan (free will compress images so quality/metadata may be affected). If having them in separate albums will be enough check iMazing maybe (guide if you'll be interested). Same synced albums as with iTunes, but easier because of UI and it won't affect metadata.

You can either use iTunes to sync your photos back to your iPhone, or preferably upload them to iCloud Photo Library. However like I mentioned in my last response, in both cases, these photos will not go back to the same place in your galley as they were before you deleted them. They will only show up as recently added photos.