Script for saving top wallpapers from wallbase.cc into a directory
Your code is nice, well documented, properly formatted and easy to understand so I won't have to much to say.


You can have a read at PEP 8 as your code is not exactly perfect for details like spacing (you should have whitespaces after : in your dicts, you shouldn't have trailing whitespaces at the end of the line, etc). You'll find cool tools to check stuff for you automatically : pep8, pylint, pyflakes, pychecker.
In save_pic, there is no way to_save != "" can be false. Indeed, because of the way you build to_save, I know for sure that it will contain a "." making it a non-empty string. If you want to be sure that the property keeps being True, you can use assert like this : assert to_save != "". Otherwise, you can just get rid of the test.
The fact that you are calling str() on file_ext in save_pic is a bit confusing : if this wasn't to behave like a string in the first place, I assume to_save = (save_path + hs + "." + file_ext) would fail.
save_pic as a function probably shouldn't rely on sys.argv[1] : I think it would be much better if this information was to be provided as a parameter. Also, to join filename, you can use os.path.join().
in save_pic (again), I think you should store the value of hs + "." + file_ext in a variable instead of repeating this in 3 different places. Also, you do not need parenthesis when assigning to to_save.
in get_pic, you could use list comprehension to define page_urls : page_urls = [r['href'] for r in soup.findAll("a") if "http://wallbase.cc/wallpaper/" in r['href']].