You can change search engine's strings in Tools -> Web Search -> Configure or in Advanced Preferences (F2): Browsing -> Finding Websites -> Search Engines
Here you can see this search string:
For Google:
http://www.google.com/search?q=
For Google Images:
http://images.google.com/images?btnG=Google+Search&safe=off&filter=0&q=
I think taht you must referer to '&filter=0' value in Google Images URL string.
Press Edit button for change the URL string value.