Hello, yes it is really weird...
I am using WordPress 3.5.1, the theme it is not the cause because i tested with several themes and the same problem, even reinstall wordpress.
When i see the problem was enabling and disabling all the plugins until yours have it.
Maybe it is because i installed the plugin on april? and if you install today and tested all is ok, but next month still uploading to May?
I don't know what is the process to upload a image, which is the php code to decide the folder...
If you want i can give you access to study the problem, i will contact you trought your blog.