On default web spiders can not read images but adding alt text tells the web spiders bot that this is actually what the image is all about.
And do you know what?
Its always advisable that when adding alt text to your blog images in a post, you should always use keywords that best explains what the post is actually about. For example if you are writing a post on Get updated information on trueinternetworld.com you can actually set your post image alt text to trueinternetworld.com information or something else that actually has the keyword in it, you Dig?
Now, How To Add Alt Text To Post Images
First, let me clarify something here. Alt text actually means alternative text.
To add a alt text, first upload an image to your blog post and click on it once
Click on Properties as illustrated in the image above, a new window appears as this:
In the alt text box, input your keyword and click on OK.
Its that simple.
hope this helps?