All web images are required to have descriptive alt text. Most web editing tools now require alt text before an image may be loaded.
ALT text is meant to provide alternative or substitute text to images. Alt text allows students who are blind to access a description of the image. When writing alt text, it is important to ask yourself what information the image is conveying. Is it definitive, descriptive, or purely decorative?
Definitive or content images are the content. A good example of this would be a location map or a picture of a cell with the different parts labeled. This type of image definitely needs to be alt tagged with a pretty detailed Alt tag. (See steps below)
Descriptive or Supporting images help the content but aren't critical to it. An example, might be a picture of Einstein next to Einstein's Theory of Relativity. For sighted users this image lends some value but does it need to be descriptively alt tagged? The best practice in this situation would be to ensure the content in the image is also present in the text content itself. If you want to be on the safe side provide a descriptive Alt tag. (See steps below)
For decorative images, leave it blank: Most sites contain a large number of images which are there for purely decorative purposes: things like spacer gifs, images forming the corner of a pretty drop shadow effect, floral patterns attached to item headers. All of these should have their alt attributes left empty. They add no extra information to the content, so as far as a screen reader or text browser is concerned they do not exist. Note: To leave the Alt Tag blank you still need to do the following steps:
Steps if using Microsoft Word 2007:
Steps if using Microsoft PowerPoint 2007: