The image ALT attribute
The alternate attribute on an image is one of the most powerful tools you can use when targeting keywords or reinforcing a single keyword throughout your page and site. An image ALT is not meant to be a long descriptive paragraph of text but rather a short concise description of the image and should take the same format as your Meta. It should clearly reflect the content of the image and not be about something which doesn’t relate at all to the image. You shouldn’t try to stuff the image alt tag with a list of comma separated keywords or similar, you could end up receiving penalties from search engines. Rather write image alts in a fluid and relevant way and the keyword matching will be done by search engines for you.
Short and relevant image ALT tags
Shorter is better for an image alt, you don’t need to include a pile of keywords and loads of irrelevant text, think to yourself; what’s the shortest and cleanest way to communicate this image to someone. Of course you don’t want to go too short; you probably don’t want to simplify image alts down to one word. A good rule is to have it about 8 to 10 words I find. If all fairness you should be able to describe and image with a short piece of text just like when you are writing a meta title or h1 tag to describe a page.
How important are image ALT's
You would be surprised just how much content you can have within image alt tags on a page if there are a lot of different images. The more images, the bigger role your image alts will play within your page SEO. You can end up with hundreds of words within your image alt tags and how you structure these can influence the flow of your page. You can wrap images in heading tags if you want and your image alt can take the role of the heading content although you should be defining your content through headings with text within and not images.
An important part of your page content
As I would always say content is the most important aspect of a web page and as technically the alt tag contains page content you should look at prioritising image alt and how they are structured within your SEO strategy.