Like I said, I was managing my onedrive folder in my computer and deleted some photos that are useless; but they're still appearing in my celphone and when I try to delete them it says it's not possible. Can someone please help me with this?
The reason you see this happens is because your phone only syncs your pictures to OneDrive when it takes them. Once you take a picture it is saved locally. At the phone's earliest convenience, it will upload the file to OneDrive. Changes to your pictures in OneDrive do not sync back to your phone.
Also if you delete a photo from your phone, the copy in OneDrive will still be there. It does not sync changes to photos, just the initial photo.
I have a Lumia 1020. I had this one very unappealing photo that kept showing up as the permanent thumbnails for Onedrive and Recent Photo under Albums. I was unable to get rid of it as it was already deleted weeks ago both from phone and Onedrive. It finally went way after I went to Storage Sense App -- Phone -- Temporary Files --- Delete. It takes several 'delete' actions to clear everything if there's a lot on your phone. Hope this helps.