You can also use the "share photo" button, select "BB Code", pick a preview (thumbnail) size. I usually edit it after pasting to remove all the extra wording flickr throws in to leave just the first url and img tags and what's between them. What's nice about it is it allows you to post smaller pics to the thread so it is easy to read, but someone can still click to open the larger image if they want to see it.

If I replace the square brackets with curly ones it looks like this:
{url=https://flic.kr/p/27gmZKu}{img}https://farm1.staticflickr.com/974/42180677102_e1585783e5_m.jpg{/img}{/url}

to get this, note that the photo is clickable:
[Linked Image from farm1.staticflickr.com]