Why choose free themes?
There are many “free” WordPress themes to download especially from WordPress website: http://wordpress.org/extend/themes/
Also there are other websites offer free WP themes such as:
and they have very nice themes, but that had released their themes under Creative Commons, rather than (GPL) General Public License.
The differences is “free” isn’t always “totally free”. Most WordPress themes that comes with some kind of condition (usually a required link on the footer area that you can not remove it legally).
I adjust/modify WordPress themes for my clients and I prefer using only “free” themes with GPL license. I use my own stock photos for the blog and I use my own graphics. Such as banners or for the slideshow images.
By doing so I do obey the laws, respect the theme owner and my clients would not get in trouble.
On the web you can find a lot of commercial themes sometimes called Premium WordPress Themes. Their license usually very limited. You can only modify within their limits. Usually they do not allow certain changes. Especially php function changes.
When you are using a theme, another important aspect is, themes almost never comes with free usage of images/graphics. All images are copyright with their creator or the template owner. So if you are considering using commercially designed themes, you must aware of the license rights. Some professional themes also offer Extended License with a higher price tag and comes with certain Resale License again with some limitations.
Commercial WordPress theme sellers:
( smashing magazine have great tips about WordPress and web design)