I figured out that the problem is due to the manual installation of PIL using pip, instead of using Debian/Ubuntu's apt-get.
So, the simplest solution would be using the
$ sudo apt-get install python-imaging
The thing is, I need PIL installed in my virtualenv environment. Getting some info from this link, I figured out a way to install PIL within virtualenv. In order to do that, some changes need to be done inside the PIL's setup.py script.
Step 0: sudo apt-get install libjpeg62 libjpeg62-dev
Step 1: get PIL source kit (here).
Step 2: untar, unzip the file
Step 3: locate setup.py from Step 2, and open that file for editing
Step 4: around line 200 (you should see other paths around there), add the following
add_directory(library_dirs, "/usr/lib/i386-linux-gnu") # for 64 bit replace with /usr/lib/x86_64-linux-gnu
Step 5: tar and gzip the modified PIL source.
Step 6: activate your virtualenv, should you need it
Step 7: pip install
I made some test with my django-blog-zinnia project, by uploading some pictures and no more error messages.