Not permitted to replace picture file in public folder

Hi

I allowed the photo upload facility in my guestbook but I want to be able to crop/compress/downsize their pictures after upload. So, I saved their pictures onto my computer, made it into a smaller file, and then I tried to upload it using FTP back into the "public" folder but the FTP application is telling me that permission is not allowed even though I have set permissions to 777 in that public directory. How do I do that?

rip