I was thinking something different.
You upload a picture as a thumbnail or full size as you said into a post which then gets published onto your main blog, which you can change the link to.
When you click on that thumbnail picture on your blog it then goes to an attachment page that is blank and has just that picture. I was wondering how to make that image on the blank attachment page full size and have it linked back to the homepage. By editing the attachment.php page.