I recently tried a site called Triberr - it's a site where you join a "tribe" of other people and share each other's tweets by retweeting, etc. When the people in the tribe post to their blogs, the site autosends the posts to Twitter. Not just for that user, but to the Twitter feed of every person in the tribe. It did indeed increase hits to my blog (Triberr calls itself "the reach multiplier"). I think for some kinds of businesses it is great. For writers, I see it as an extremely valuable tool.
However, I received messages from some Twitter followers saying that they missed the "personal nature" of my Twitter account as it was before. A couple tweets went out automatically that had some kind of questionable titles and/or content.
Between the kind nudges from the fans and the inappropriate tweets going out, I decided it wasn't for me. Not to be prudish, it wasn't so much the content as much as my feeling of lack of control of things being posted that looked like I was posting them but I wasn't actually creating each action. I really don't feel comfortable in having that control taken out of my hands.
I will say though, that the networking and friendship factor in Triberr is a great thing, and one time when I had a problem, one of the creators of the website (Dino Dogan) emailed me personally and fixed my problem for me. I was very impressed! So Triberr has a lot of great things going for it.
I'm not knocking Triberr, but I do encourage you to really think about it, whether using a tool like Triberr to auto-tweet is going to be helpful for you, or whether it won't be a good "fit" for a portraitist. Personally, I think that portraits are such a personal thing for people to order, it deserves the personal touch.
If you want to learn more about how Triberr works, check out this About Us page of Triberr's blog.