I have a string of code from Authorize dot net that I wanted to add. That string of code will show their logo, with a link to our "Merchant Verified" certificate. But they don't give you a logo png. Instead, you get a string of code to strip into your site.
Using the left-hand menu, Navigate to your Admin/Online Store/Themes/"whatever version of your theme you are currently using"/click on "Actions" button and select "Edit code"
Warning.. this is for sectioned themes. I have no idea how to do this on a non-sectioned theme. Sorry.
Use the menu on the left-side to navigate to the "Sections" folder. What you are looking for is the code for the "footer" of your website. (Thats the part at the very bottom that has all that neat stuff like links to your "contact us" page and maybe your newsletter button, etc.)
It will say something like "footer.liquid".
You will be dropping in that string of code you got from Authorize dot net in here. WHERE you put it is up to you.
I wanted ours to display right under our list of links displayed on the far right of our footer.
Here (marked in red) is an example of where I placed that string of code ...