Here is how you can add alt text to your images. If this is the image code -
<a href="http://2.bp.blogspot.com/-TADZiK7-x-k/TtbnFClFkZI/AAAAAAAAGSU/8icT7geB2Ew/s1600/lightroom_advanced_photo_blogger_template.png" imageanchor="1" style="margin-left:1em; margin-right:1em"><img border="0" height="328" width="400" src="http://2.bp.blogspot.com/-TADZiK7-x-k/TtbnFClFkZI/AAAAAAAAGSU/8icT7geB2Ew/s400/lightroom_advanced_photo_blogger_template.png" alt="Lightroom advanced Photo Blog Template" /></a>Insert alt at the end of the code and add your image description. This will also help to generate image traffic if you manage to include the right keywords in image alt and at the same time the reader should understand the image content. The text doesn't need to be very long or spammy.
Also while saving or uploading an image ensure that you have renamed the image with the keywords and not just saved it as 'IMG 45672.jpg', this is an effective SEO tip to attract Google image traffic.
In case you are linking the image to another site or content on your blog, then you can add title text after alt, title text would be a short description of the link or the image. Please note that Google uses title text only if the image includes a link.