This is one task where I greatly prefer using Bing to Google search. Just go to Bing, then click on “Images” on the top toolbar. Enter your search term, and when the results come up, you should see a list of options above the results (size, color, type, etc.). Click on “license” and select the appropriate usage license from the drop down menu. One of the options is free to modify and use commercially. Bing will filter the results accordingly.