a11y - Challenge buttons go away in Windows high contrast mode

[GoogleCodeExporter]

Fully patched Windows 7 Firefox 38.0.5

Toggle Windows high contrast mode on with left-alt+left-shift+printscreen. 
Reload page with recaptch and check the not a robot checkbox. The new 
challenge, audio and help buttons disappear because they are constructed using 
css background images. Background images are used for decoration, not content, 
so they are considered disposable, which high contrast mode does to de-clutter 
the display. Change to standard img tag sprite maps instead to fix.

Original issue reported on code.google.com by [email protected] on 4 Jun 2015 at 3:00