When you use images on your character sheet, they get filtered through a Roll20 url. Your <a href="https://sites.google" rel="nofollow">https://sites.google</a>... address becomes <a href="http://imgsrv.roll20...?src=sites.google" rel="nofollow">http://imgsrv.roll20...?src=sites.google</a>... Specifically, the filtered url for your image is coming up with a code 302 (Moved Temporarily), so the image won't render when the character sheet is displayed. This is not normal behavior, as it doesn't occur for background images on other sheets. I can only speculate that it's being caused by the fact that your original image is on a secured connection. Try uploading the image to something like imgur.com: some hosting location that you can get an HTTP link, rather than an HTTPS link (all Google Sites pages will force you onto an HTTPS connection). See if that solves your problem.