Apple Affiliate program offers a number of powerful tools to fully integrate store links and content into your blog. They include: Link Maker, auto Link Maker, Widget builder and Banner builder. You can easily access them via the "Tools" tab once you sign in to your account.
If you want to generate ad banners of different sizes, then you need to use the Banner Builder tool. Banner Builder lets you generate banner art instantly for nearly all content on iTunes, the App Store, the iBookstore and the Mac App Store.
Below of a sample of what the 300x250 ad banner looks like:
How To Use Apple Affiliate Banner Builder
==> Sign in to your Apple Affiliate account
==> Click on "Tools"
==> Click on "Banner Builder"
Now, lets assume you want to generate 300 x250 ad banner to promote Wizkid Album on your blog,
==> Enter "wizkid" as search term
==> Select "music" from the "Media Type" drop down box
==> Click "Search"
==> You should see Wizkid albums and songs available for you to promote
==> Click on the Music Album you want to promote eg WizKid - Ayo music album
==> A new page will open where you can select the ad banner size you want as highlighted in the screenshot below. You will also see the preview of the ad banner on the page.
==> Copy the code and place the code on your blog.
I hope this helps.
NOTE: You can generate ad banners for nearly all content on iTunes, the App Store, the iBookstore and the Mac App Store. So, don't limit yourself to promoting only music.
If you've got questions, relay them via comments.