Re: Same filename, different category.
Images are kept according to your userid, so when you upload, it all goes to the same folder.
Image names are not stored intact. Everything beside "a-z", "0-1", "." and "_" is deleted from the name.
And finally to answer your question =) no, I don't think images are overwritten (they weren't in RC2). They are suffixed with a 4 letter numerical string.