Excellent. On Steem, we very strongly encourage sourcing all material borrowed, even when not required by the original supplier. You can always put those sources at the end as you did with your other sources. Also, it's very easy to have the "caption" be one word - source.
That's easily coded as:

[ source ] ( url of image/text/whatever ) - done. (Take out those spaces I put in to keep it from working in this example.)