MGNLSTK-494 added the possibility to have the ThemeUpdateHandler install 2 images in your theme "icon.jpg" + "icon.gif" with the same file extension (see info.magnolia.module.resources.setup.InstallTextResourcesTask).

InstallTextResourcesTask(String taskName, String taskDescription, String encoding, String pattern, String template, boolean processed,
            String modelClass, boolean preserveResourcesParameters, boolean stripExtension)

The last parameter stripExtension would have to be set to false, to "enable" the new feature.

See:

• info.magnolia:module.templatingkit.setup.ThemeVersionHandler.java:61
• info.magnolia.module.templatingkit.setup.ThemeInstallTask