Ok, reno heres how to do it, its very simple...
Open the 'My.html' file with windows notepad
Look in the text for the name of your image file, you will see it in this line of text near the bottom...
<img src="
Property Sticker-01.jpg" id="bg" />
Now simply change this to the name of your new image & save, job done!.
Ps: If its just an image you want to change why bother with the html ?, why not just put your image/s in the 'Advertising Images' folder ?.
Steve