If I remember it right, this happened to me once. I deleted my /uploads directory (to delete all the images) and went back to posting thinking that WordPress will automatically create an /uploads directory and any necessary sub-directories for me.
And yes, that’s how it works. But here’s the catch — WordPress doesn’t set proper permissions for the /uploads directory and the directories within, this time. The new chmod permissions were like 0700.
SOLUTION: What I did was, I set back the permissions for /uploads directory at 0755, and selected the option in my FTP client to set the permission recursively to all sub-directories in the /uploads directory.
As in your case, I would also check the permissions on /wp-content directory.
