If you want to support xbox controllers, it is always wise to use the native images that the controller uses. Can one obtain those from Microsoft without being a registered, aka paying developer?
You can get official Microsoft logos here: http://www.microsoft.com/en-us/news/ImageGallery/logos/logoguidelines.aspx (You have to agree to their terms in order to get to the logo part).
Here you might get some publicly available assets: http://www.microsoft.com/en-us/news/imagegallery/products/default.aspx (Well, there isn't a top-down shot of X-Box controller, but maybe you can get away with Limited Edition one)
So, to directly answer your question - no, there are not publicly available official images to be used for keymapping hints.
Last, but not least, you can always try to look up contacts of the company, and send them an email. You're not asking for a free X-Box, Kinect, TV and sofa - it's only an image.
I, personally, am not a developer of X-Box games or anything related to X-Box, but maybe you get some illustrations with the developer kit?
There is an unofficial but extremely complete set of controller button images for several platforms here: