Question: As stated in [Dropbox FAQ](https://www.dropbox.com/help/288/en):

> 
> Camera Upload tries to be smart about not uploading the same photos
>  and videos more than once. It will not re-upload these files even if
>  you move, rename, or delete them from the Camera Uploads folder.
> 
> 
> 

So, how can I clear this "cache" and reupload all photos again after deletion?

Answer: 
I don't think Dropbox will let you do that. The Camera Uploads is also how they keep track of how much you have uploaded. They give more space to you the more you upload. If you could reset that counter then you could max out your space very quickly, something I am sure they have thought about.
Try to delete the photos from the trash bin after deleting them from your photo uploads folder and, after this, re-upload the photos. 

If still not possible, use a different file name and when it's about hundreds of photos use something like a "bulk rename utility."