I just tested this out on my test blog (http://ellietestblog1.blogspot.com/) and it seems the way to do it is to go to your dashboard > Design > Page Elements. Then there should be a box under your header where you can add a gadget. Add the HTML/Javascript gadget and then use HTML to customize it.
To center your images/links, put them between these tags:
HTML Code:
<center></center>
Image tags in HTML should look like this:
HTML Code:
<img src="YOURIMAGECODEHERE">
To make your image linkable it should look like this:
HTML Code:
<a href="YOURLINKHERE"><img src="YOURIMAGECODEHERE"></a>
Let me know if you need any help!