Blogs aren't with forums or wikis or public noticeboards. A blog is web site for a specific person or a specific group to post articles and visitors can comment on them. In a blog the control of content and value is driven by a single person or small group. A discussion board is used to solicit feedback from others by generating dialogue. Anyone can post a message and users can respond to each other. On a discussion board control of content and value is shared equally across all users.
On a blogm if you want clients to be able to post to your blog, then you have to invite them to become official users of your blog and they have to accept the invitation. Then you add them by designating the appropriate user role.
Inviting Contributors, Followers, and Viewers > http://en.support.wordpress.com/adding-users/
Alternatively, if all you want is the images and testimonials from your clients they can post them into comment boxes or into a contact form including image URLs and then you can post them to your blog on a testimonial page.
Related link > http://en.support.wordpress.com/comments/embeds/