How you do it:
Go to where you see the html for your post. For the image above, the html looks like this:
< a href="http://belladia.typepad.com/photos/uncategorized/2007/11/25/img_4633.jpg"img width="350" height="262" border="0" src="http://belladia.typepad.com/bella_dia/images/2007/11/25/img_4633.jpg" title="Img_4633_2" alt="Img_4633_2" / > < /a >
The tag in red is the title tag.
Between the quotes is where you put the information that you want to show up when someone puts their cursor on your picture. So, if I want to say, "Toulouse between the sheets" then I put that info between the quotes like this:
title="Toulouse between the sheets"
Try the picture now... Got it? Easy peasy!
Just go slow, make sure you don't lose the quotation marks, and have fun. It's a great way to give a little information about your picture.
Edited to say thank you to Alice for code help in making this post easier to read - Thanks Alice!