ADDING ALT TAGS IN DREAMWEAVER
Be sure you're in design view (click the 
design view button, usually in the upper left).
Find and right-click on the image you wish to add an ALT tag to.
Select "Edit Tag <img>..." from the menu that pops up.
On the next screen, you will see a place to type some text that will appear on the screen instead of the picture if the picture is unavailable. You should see something like this:
When you've finished typing, hit "OK." You have now added your very first ALT tag.