Hello celestefromariccia,
Have you tried importing the pictures from your iPhone onto your computer? Usually at the end of import you are given the option to delete them from the iPhone. This affects only images that were taken with the iPhone.
Import photos and videos from your iPhone, iPad, or iPod touch to your Mac or Windows PC - Apple Support
http://support.apple.com/en-us/HT201302
This section will explain how to remove any pictures on your iPhone that were synced from the computer using iTunes.
iTunes: Syncing photos - Apple Support
http://support.apple.com/en-us/HT201313
- In iTunes, click the Device button in the upper right corner. (If viewing the iTunes Store, click the Library button in the upper-right corner first.)
- Click the Photos tab in the resulting window.
- Choose "Selected albums" and deselect the albums or collections you want to delete.
Note: To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
- Click Apply.
If you need to delete any other images from your iPhone (email or text attachments, etc.) this article will explain some strategies to free up additional space.
See how much storage you've used on your iPhone, iPad, and iPod touch - Apple Support
http://support.apple.com/en-us/HT201656
I hope this information helps ....
- Judy