I usually use Sass and Compass to give my application a custom theme.
If you are using Sass you con do that in this way:
Where $theme-name is a variable which contain a value of "default" by default.
-webkit-mask-image: theme_image($theme-name, 'your_disclosure_pic.png') !important;
In this way, you only need to put your "your_disclosure_pic.png" inside the "themes/images/default" folder and compile your theme.
(You can even set another variable name to point to another folder different from the default one),
If you want do that directly using CSS, you can do the same defining
Hope this helps.