File System Public upload picture problem

Ok so I want to use the public file system and I made my own paths

file system path: pictures

and temporary directory: pictures/temp

The thing is that when I upload an image it works and it is uploaded to the temp path.
BUT when I try to change my picture, it get's uploaded to the temp path, but the file itself (in picture path) is not changed.

It is still the same file that is in the map pictures (called picture-1.jpg)

So how can I make sure that this file changes? Or even better: how can I make sure that under the picture map a file is uploaded with his real name instead of picture-1.jpg?

Thanks!