In short, if an image contains significant text, that text should be used as the alternate text. If the image is a photo, a drawing, a chart, or a map then a short description should be given of it. Otherwise the image probably should not have any alternate text. Perhaps a couple examples would help demonstrate what I mean.
Let's say you have a navbar and your client wants a specific font used in it. So you put an image in each link instead of just plain old text. The alternate text for each should simply be the text that is embedded in the image.
One incorrect usage that I see many people using is alternate text like "top right rounded corner". That is just plain wrong. If someone can't see an image of that kind they have no need to know it's there.
So please give some thought to how you provide alternate text for the images you include in the pages you create.