Are these attachments (which utilize the specified upload folder) or are these images for a gallery forum (which would utilize the /gallery folder; which would have its on permissions)?
Are all of the fields set in the Attachments settings (CP -> Master Settings -> Attachments)? Can you submit the attachment settings page in the control panel and attempt to upload again? (when you submit the page without changing settings it'll write any defaults that could be missing from your configuration file)
