Cannot Upload.

Forumophilia should address this issue. I realize that many people have posted about this issue, but no one says why it happens. I have heard of lots of people (including myself) who try to upload an avatar from their machine and the get the following error (or similar):

Warning: copy(./images/avatars/67840281943283885056e0.jpg): failed to open stream: Permission denied in https://www.forumophilia.com/home/zorgefhgo1/forumophilia.com/includes/usercp_avatar.php on line 241

Didn't bother me, I just found an avatar with it's own URL, but some people wish to use their own images. Why is it that images cannot be uploaded?