Simple PHP custom code

I was wondering if someone could help me out on a simple custom PHP code I need for my wordpress blog.
All my posts on my blog have a different image in the top left that goes to the same external link.

Currently if I want to change that link I have to update all 200 posts on my blog individually.

I was looking for a snippet of code that I could put into the singlepost.php file (or any other way) that would allow me to put this external link in one place and all my posts would be updated.

Thanks!