This isn't a PHP limitation, but rather a *nix "limitation" - and with good reason. A user that doesn't have permissions to delete a file shouldn't be allowed to delete that file!
You could try creating a new FTP user that is a member of the same group as Apache, and only using that user to upload the images.
You could also just change the permissions of your source files after you have uploaded them.
Or you could look into the dangerous
exec() and
system() functions, which you could probably use to temporarily elevate permissions while you delete the files.