When someone creates an image they have the right to control how that image is used. For example they might say "For non-commercial reuse", which means you can use the image but not for any business purpose, or they might say "For reuse with modification" ie. you can use the image for commercial purposes and you can modify the image.
Google now supports searching for images based on usage rights. Please follow the steps outlined below;
1. Type your search keywords in the Google search box & click the search button (the magnifying glass)
2. Click the Images link
3. Click the Search Tools link
4. Click the Usage Rights link
5. Select the Usage Rights, from the drop-down, that are appropriate for your situation