Creative Commons states that the most complete citations 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
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
Image Citation Guide: UBC's guide has a number of examples based on different media sources and citation styles.