It is common that on and off we are using someones Photogrpahy or work to create our own. I did few and used the photos of users on Steem, but I asked their permission and they agreed. If you have the same simply paste their permission in your post and also tag his name. Many will be happy with that and even come with their comment too.

If the source is outside of Steemit then see if the source is open, if not then of course it is the case of copyright.

Please let me know if you need more help :)