Problems with subdirectories when using a WikiPage skin

• create a new skin in a WikiPage (object XWiki.XWikiSkins)
• use xwiki10b1 as baseskin (a XWiki.XWikiSkins field)
• configure your wiki to use this skin
  -> the images stored in xwiki10b1/icons/ (like blue tabs and buttons in the edit mode) does not appear.

The normal behavior in xwiki is to fallback on the baseskin (on filesystem) when the file (ie: an image) is not present as a WikiPage attachment, this seems to fail when the file is originaly stored in a skin subdirectory (like xwiki10b1/icons/).