what kind of blog ?
If it blogger ,use this method (
Sure you can put three 125 x 125 banner after each post. Your banner won't appear until people click on your post. I guess it is a win-win solution, people read your post and then you show your banners.
The problem with this is it'll probably affect your overall blog design unity.
Open your setting and go for edit HTML, put a tick on expand widget(it is on the right top).
Find this code (You can use ctrl+F in Firefox or command+F for Mac user),
<b:include data='post' name='post'/>
And add the following code
<a href="your-url"><img height='125' src='image_url' width='125'/></a>
<a href="your-url"><img height='125' src='image_url' width='125'/></a>
<a href="your-url"><img height='125' src='image_url' width='125'/></a>
I put 3 because I find that your blog post can only fit 3 banners.
Try it, if you don't like it just remove the code you have added.
)
if it wordpress use this method (
You can use thus plugin
http://wordpress.org/extend/plugins/all-in-one-adsense-and-ypn/#post-2664)
Thanks