Hi Andria,
You can override the path for the button image on the AddThisButton control using
ButtonImageUrl="~/pathtoyourimage.png"
where ~/ will be resolved as the root of the site.
Some controls can contain other controls. InsecurePanel is really a custom version of <asp:Panel which is a container control that renders as a div. All InsecurePanel is for is to suppress insecure content on a page that is secured with SSL. The reason the AddThisButton is inside it is beacuse it does some non-ssl stuff and if you let it render on a page secured by SSL the user will get browser warnings about the page having both secure and insecure content. So InsecurePanel just detects if the page is using SSL and if so it hides itself and any contained content, so in this case the addthis button will not be shown on pages secured by ssl to avoid the browser warning.
Hope it helps,
Joe