If you are going to use photos (or other media) on social media, use public domain materials or the ones published under CC0 license. If you want to use materials published under other free licenses, you will have to ask for permission from the authors, due to the restrictive nature of Facebook, Twitter, and Instagram's Terms of Service. You can read more here.
The text of the permission, for example, for the pictures uploaded during Wiki Loves Earth photo contest is below. Ask the authors to forward the text to digitalmediawikimedia.org, if you are going to use it for WMF social media:
I consent to the use of the photos that I have uploaded during Wiki Loves Earth photo contest on Twitter, Facebook, Vkontakte, Instagram, Google+ etc.
You can also ask for the permission to use a certain file:
I consent to the use of the photo [insert the link to the photo] on Twitter, Facebook, Vkontakte, Instagram, Google+ etc.
If the authors do not support social media, you can ask to permit the photo with lower resolution:
I consent to the use of the photo [insert the link to the photo] up to a size of 1024 x 385 pixels on Twitter, Facebook, Vkontakte, Instagram, Google+ etc.
Do not forget to inquire how the author(s) want to be attributed. Maybe they would love their real name to be mentioned, not a username