To link to a source, you need to find the the link for the page the original image came from, rather than the link for the picture itself. Put that under the picture link.
To get it tidier, you would put the text you want to be the link eg Image Source in square brackets like this [ ] and then you'd put the link itself in round brackets ( ).
To read more on that, have a look at @thecryptofiend's article on Markdown (linked to in my post