A good rule of thumb is to recall the acronym TAL, which stands for Title, Author, and License.
Title and Source: "The Mountains Are Open" (linked to image on Flickr)
Author: Alan Levine (linked to profile page)
License: CC BY 2.0 (linked to license deed)
“How to Attribute a Creative Commons Licensed Work?” Attributing OER, Open Education Consortium, www.cccoer.org/attributing-oer/.