see: 7116127, which describes this problem while only fixing the case where both jnlp file and image are updated.

an ico file is generated by IcoEncoder.java when one of the image files is being used for a shortcut.
The file is tagged in the cache entry with a shortcutImage flag, and the clenaup thread then knows not to remove it.
But if the original image resource is updated, you get to removeCacheEntryImpl(), and the image file along with any ".ico" file generated from it are removed.

The "default" image file is treated like a resource. that is, LaunchDownload.downloadEagerAll() will download or update it from prepareResources(), if update mode is background then LDUpdater will do the same thing.
in either case the .ico file previously generated for a shortcut will be removed, and the shortcut will point to a non-existant file.