Citing Creative Commons
Creative Commons states that citations should include the Title (if there is one), Author (or owner of the material), Source (where it can be found; usually a link) and License (the name of specific CC license with a link to that license). Copyright holders may require that a specific citation style or information be included. On the CC website, see: Best practices for attribution and Attribution in specific media.
With hyperlinks (ideal)
"question" by cesar bojorquez is licensed under CC BY 2.0
With hyperlinks (short)
"question" by cesar bojorquez / CC BY 2.0
No hyperlink
"question" by cesar bojorguez (https://www.flickr.com/photos/uncut/16926192/) is licenced under CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/)
With license icons (short)
"question" by cesar bojorquez 
Remixed / Modified work
"Thinking" by Freddy Alequin is shared under a CC BY-SA 2.0 license / Modified from original and shared under a CC BY-SA 2.0 license
Citing Other Licensed Works
For non-Creative Commons works, check to see if the creator or website states how images or media should be cited.
The Noun Project, for example, requires a specific statement and format. See: How do I give creators credit in my work? and Medium-specific credit requirements and examples.
With hyperlinks
"Pisa tower" by Danil Polshin from the Noun Project
No hyperlink
"Pisa tower" by Danil Polshin from thenounproject.com
Further Help
To use others' work legally, it's important to follow the requirements of the license or any terms of use. Attribution is a very common requirement.
When you give credit to the creator of the work you're using, you don't have to use a specific citation style but you do have to provide attribution in the way they've requested. Some licenses may require that you place the citation near where the work was used, while others will allow you to group citations together at the end of a work (e.g. end of video; last slide of a PowerPoint).