People say it adds credibility, and I don't doubt that. Sometimes, though, even if you post your real picture people don't think it's you anyway (because so many people don't post real pictures). Then, too, there's the matter of people who write personal experiences. My original reasons for the picture I use were that I didn't have a good web cam and don't have a scanner. Besides, I thought my writing should stand on its own, separate from my appearance. These days I'm pretty sick of the picture, but I'm concerned if I change it, it could affect my earnings.
I chose it because it's the Earth - something all people have in common. It's a pretty picture, so it "applies" to both sexes but has a feminine flavor to it. It's transparent - something I like. It's also generic enough to go with a variety of subjects I write about. It's been up so long it's kind of starting to look "lifeless" to me, though, so I may see if I can come up with a real picture at some point.