Creative Commons (CC) Licenses allow you as the user of copyrighted material to use materials as the creator intended or allows. Copyright law "reserves all rights" to the copyright holder. Creative Commons License is set by the copyright holder to allow other people to use their work, based on the copyright holders preferences. You do not have to contact the copyright holder for permission to use their resources when they have provided a CC License for the work.
You MUST provide attribution to any Creative Commons work used. TUAL is an easy way to remember how to do this.
Title of work (URL) by Author, License
You will Hyperlink both the Title and the License so that others will know where to find the image and be directed to the full license.
The creator has not given permission for you to use this work and make money from \your new work, nor can you change the photo.
On your poster, the image would look like this with the attribution just below the photo:
You can search for images that are free for your use in several sources because the owner has offered them for your use under a Creative Commons License.
Use the "Any license" drop-down filter to limit your results to the appropriate license
In Flickr, click on "Some Rights Reserved" to see what you can and cannot do with the image or photograph.
When Using the Google Image search, click on Tools and then Usage rights to locate images that have been labeled with a "Creative Commons License".