And in this post i will show you exactly how to add a banner to your blog (like image bellow)
So the first thing to do is choosing your banner ads , you can use google or create your own banner using photoshop for example .
For the size : if you want to add the banner in the header of the blog i advice you with 728x90px but if you want to add it as a gadget the best is 125x125px .
The seconde step after choosing the banner is upload it in a new post and add the code HTML in a new Gadget .
You can follow this video to learn more .