keep in mind what follows only apply when preparing posts in the EDIT HTML mode and not in the COMPOSE (WYSIWYG) mode.
I prepare practically all posts in the [Edit HTML] mode and you are highly encouraged to do the same. You get better control over your blog. HTML is not difficult, and as you browse through my Blogger related blogs (this blog as well as the blogs in my signature line at the bottom of this post), you will eventually pick up the more important elements of HTML. I guarantee you it will not be difficult. If you are more familiar with the COMPOSE (WYSIWYG or What You See Is What You Get) mode, don't fret. You can always check by clicking "Preview" and get WYSIWYG displayed, just as in the COMPOSE mode.
A typical script you will get after uploading the image is given below:
you will get when you upload an image like the one below (Update: If you use the updated post editor you will have to add the alt="" tag yourself, otherwise use the old post editor to upload images):
Note the alt="" highlighted in red. This is a HTML attribute which had been added by Blogger for your convenience. You are greatly encouraged to put some text between the quotation mark "", like for example, alt="some text" where "some text" are text which is descriptive of the image. For example, for the image above, I have edited alt="" to alt="Blogger Dashboard: 72 comments waiting for moderation".
The purpose of this alt attribute is so that if for some reason the image cannot be displayed, the ALTERNATIVE TEXT that you have put between the quotation mark in the alt="" attribute will be displayed instead. It will also be of the help to the visually handicapped as the alt text will be read out for them. Also some visitors may be using all text browser which will see the alt text instead of the image. Further, search engines which cannot read images will be able to read the alt text an idea of what the image is about.