When you create a Web site, you should be thinking of accessibility so that it makes sense to people using screen readers or just browsing with images off. The easiest way to make sure that your site makes sense to them is to turn off your images. When you do that in your browser, the alternative text in your images should appear in place of the images.
With the images off, read the Web site. Does it make sense?
If you're building pages like most designers, you've probably got at least one or two images that are actually text. While I don't recommend using images in place of text, there are sometimes design reasons for doing so. But that's when the alt tag really shines.
The alt text should say exactly what the image says - and nothing else.
Think about it. When a reader arrives who can see the images, she isn't thinking "ooo, that's a 300x40 pixel gif with a white background that says 'Joe's Pizza'", she just reads "Joe's Pizza" and moves on. Why are you forcing more information on your visually impaired customers?
Yes, if the image is something other than text, you want to describe it. But even then, you should be describing what it's a picture of rather than the technical specifications of the image. Your customers don't care that you used Photoshop CS to build the picture or what size it is, they just want to know what they are missing.
I see this all the time when I browse with images off:
"Joe's Pizza Logo"
This is similar to providing technical details about the image, but it's more corporate details. Logos are often placed on a Web page in such a way that when the screen reader reads it, it's obvious that it's a logo. Why would the reader say "About.com Web Design HTML Computing and Technology ..." at the top of the page if it weren't reading the site logo and branding.
If you're worried that your visually impaired readers won't realize that you're trying to brand your site with your logo, well, maybe you have a point. But frankly, I would argue that if you don't have a strong enough brand in your writing that you must have the word "logo" sprinkled in whenever you place the logo, then you probably have other branding issues that go beyond what a logo can fix.
Alternative Text is an Alternative
If you remember that writing alternative text is supposed to be what people see if they don't see the image, then that can help you write better alt text. Your readers will thank you.