Over the past few days there have been complaints in the Google Web Search Helpforums that the advanced image search feature has gone missing.
In fact, Googler, Jeff, said Google is aware of the issue and is investigating it. He said:
We are aware of this issue and are currently investigating. I’ll post back here when we have more information to share.
I personally see it myself under the gears icon at the top right corner:
If you do not see it, try going directly to google.com/advanced_image_search.
I am not sure if it is a browser specific issue or if it is resolved already.