Facebook is probably fine since you can grab an external image share URL, and maybe the other social media accounts (not sure), but it is generally considered bad form to hot link images so if you are trying to use an image hosted on someone else's website you would be better off downloading it and re-uploading it to imgur or another image sharing service that allows this. For a small photo I doubt you would realistically get in any trouble over the little bandwidth you'd use, but still.