Dreamweaver 8 - Adding ALT Tags
If you have Dreamweaver 8 ALT tags are a breeze. When you add a picture a small box will appear:
Enter your alternate text in the "Alternate text" box. If you description of you image is especially long you can create another page and link it in the "Long Description" box.
Dreamweaver MX & below
Hit [ CRTL-F3 ] and the properties dialog box should appear. Click on your image and on the top row of the properties their will be a box for ALT. Enter your alternate text here.