phranque - 9:54 am on May 21, 2013 (gmt 0)
welcome to WebmasterWorld, danagraphics!
since yahoo search is powered by bing, you can only do this (as the webmaster) using the Bing Content Removal Tool in Bing Webmaster Tools:
if it's an issue of copyright infringement you can submit a Notice of Infringement:
you might get some action from the webmaster if you threaten to file a DMCA take down with the search engines, but make sure you're on solid ground before doing so.