You should be fine as long as you only allow certain file types to be uploaded. For instance, you don't want people to be able to upload files that may contain malicious code to your website. You will want to check the file type of what is being uploaded to ensure that it is an image. If you've already done that, you shouldn't have any problems.

let's say you have uploaded "abc.JPEG" on that directory... if someone knows the directory name and the "abc.JPEG", they could just directly type that on the URL and they get to access "abc.JPEG".

Well if that were the case, you could write a .htaccess file that only allows requests from localhost to access the files in the directory.