When you use something that has been licenced under Creative Commons you are legally obliged include information about the licence that is being used. If the licence permits you to make a derivative work or adapt it, you must state this. You should:
For example:
Figure 2: 'Finish/Start' by Anne I Like from Flickr, CC BY-NC-ND
Finish/Start' is the title of the work (http://www.flickr.com/photos/ilike/3707503212/). The author is Anne I Like (http://www.flickr.com/photos/ilike/). The site is Flickr. The licence is CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/2.0/deed.en_GB).
In this example "CC BY-NC-ND" indicates that the image has a Creative Commons license (CC), where the author must be attributed (BY). The work is only for non-commercial use (NC). The work cannot be changed in any way (ND - NoDerivs).
Creative Commons license conditions are usually shown by icons. The icons for "Finish/Start" are:
Figure 3: Creative Commons icons
The "Some rights reserved" link opens more information about the licence. Always read the information carefully to ensure you may use the image in learning and teaching materials.
For more information see the Creative Common’s Best practices for attribution: https://wiki.creativecommons.org/wiki/Best_practices_for_attribution