The following works at the school I support. From the user guide:
"It is possible to stop Google image searches but the only way to do this is through the use of 'Regular Expressions'.
The regular expressions created for this purpose must be added to the Local Block list of each policy where Google image searches should be blocked.
The quick method of doing this is by going to the Local URLs and Keywords page and clicking the New URL or Keyword button.
Then add the regular expressions listed below, one at a time, and specify which filtering policies they should be included in.
Please copy and paste the following regular expressions into your filtering policies (they must appear exactly as shown with brackets, etc):
(www\.google\..+/.+\&site=imghp)
(www\.google\..+/images/)
(www\.google\..+/imghp)
(images\.google\..+/)
(tbm=isch)
(tbs=img:1)
(tbs=isch)
https://support.atomwide.com/secure/guide/WebGuide/Help.html?ws2_faq.html
For this to work though I would expect that the LGfL DNS would need to be used.
I assume with Yahoo, I haven't tested it, that you can simply keyword block images.search.yahoo.com
Hope this helps?