Google Images has offered the option to search for images available through Creative Commons and other free-to-reuse licences for a while, but you had to head to advanced search to find them. A revamp of image search means it's now easier to filter out pictures that can be reused or modified.
On the images search result page, click on 'Search tools' and then on the 'Usage rights' drop-down menu that appears. From here, you can choose to see only images that are licensed for reuse, or for reuse with modification.
If any kind of advertising is going to appear where you plan to use the image, be sure to choose the options for "commercial reuse". In most cases, you'll still need to identify the source of the image when you reuse it, but this makes it a lot easier to find images that aren't going to be the subject of a copyright dispute.
[via Matt Cutts]