While alt tags are important, make sure you focus on getting links to your site, copy keyword rich website copy, and that you have a good internal link structure. These will have much more impact on SEO and traffic down the road.
tssdetailing brought up another good point in his post, making sure that your links, copy, and meta information is location specific. For keywords like detailing you will not rank well. A lot of things go into ranking, so for time I will skip that aspect. However, it can be quite easy for your to rank well for Greenville car detailing. Which for our business gives us better leads anyways because most of us have a small typical working area instead of nationwide.
Again having meta information, links from other sites to yours with location specific link text, copy on your site, and internal links with keyword rich copy is going to help you rank well and lead to more traffic.
In the way of links, I have town specific business directories help local based search rank quite a bit. Even if it costs money, a link from one of these sites can really boost your rankings.
I would also look to create a Google local business page here:
[url]http://www.google.com/local/add/analyticsSplashPage[/URL]
For your headlines and copy make sure that you are communicating the value of your service. A good example if your homepage. One of the headlines read, " Our most popular package starts at $150." How about, "Add hundreds if not thousands of dollars worth to your car, our most popular package starts at only $150". This difference in the two statements is the latter informs the perspective client of the low price, but also the value in spending their money on detailing. Also, for people weighing if the money is worth it to spend, you are helping that decision process. You can also appeal to people differently. For example a headline like, "What to make people stare at your car in amazement? Our most popular package starts at only $150.
On the before and after page your examples look great, however you are not using the page and the emotions it stirs to the full benefit. What I mean is for some users they are going to arrive to that page and see the pictures and think to themselves,"Man this looks great I want my car to look like that as well." Visitors reach the bottom of the page, and they are left to their own devices to find your contact page, book an appointment, or call you. Instead provide that information for them on the page, so you are capturing them when excitement is most. Something like, "We can transform your car just like these! Call us today at: xxx-xxxx". Online a lot can happen in the time a visitor take to look at the before and after, and then proceeding to the contact us page.
One thing I see lacking in the navigation is testimonials and awards. For the detailing business do not must of your clients arrive to you because of word of mouth? In that case provide a similar feature with some testimonials of your work from satisfied clients. It is also a good idea to sprinkle them throughout the site to continually add credibility and reason to decide on you. In the way of awards, if any of the cars you detail ave won any awards then showcase that on the site.
Hope that helps and let me know if I need to clarify anything.