A Microsoft file hosting and synchronization service.
Mathieu,
Thank you for using Microsoft products and making community posts.
I understands the importance of resolving this issue, Don’t worry I will be helping you with this following steps.
Did you sign out your OneDrive account before deleting those photos?
Take note that if you will be deleting your pictures on your gallery without signing out your OneDrive account it will be also deleted on OneDrive since your Iphone and OneDrive still synced.
Try to check on the recycle bin of your OneDrive account and take that deleted files will be permanently deleted from OneDrive after 30 days.
Please feel free to let me know how it goes.
Chan G
Microsoft Community Moderator