Not necessarily – it really depends on your site.
For example, if you have a business training dogs or running workshops, fly-on-the-wall shots taken with your mobile often give more authenticity that professionally posed shots.
But by and large, if you’re selling a product or a business-to-business service, they can make or break your site.
I recently built a site for a lady who designs and makes jewellery (I am writing this with her permission) and she gave me photos which she’d taken herself with her mobile. They were, not to put too fine a point on it, pretty awful. She took the criticism in the spirit it was intended and had the photos taken again by a professional photographer, Carl Parris. Have a look at these two and compare the difference: