You will need access to the source code for sure. But the process works like this. Save the image to your hard drive, upload to your hosting account, link to it in the HTML with
then upload your new html file as well.
I dont know what the banner is (like if it is supposed to link to your actual certificate, etc) but if this is the case, it requires a little more. Again, you need to access the source code regardless.
Contact the company that built the site and they will probably do it for you (for a fee). Or, if you aren't tied to them by a contract of some sorts, you can get someone else to do this for you. Hope this helps.