To link photos on Flickr ...
1. Click on the photo in your photostream
2. At the bottom-right there are 4 icons. One is a curved arrow thing that says "share photo" when you hover over it. Click on that
3. A popup will appear. Select the "BBCode" on the popup if it isn't already showing by default. This gives you the code you need to copy to show it on a forum
4. Copy the code in the thing that starts [url=https:..."
5. Please do not select the largest size. Really large photos are annoying for those of us who aren't using a cellphone or tablet. The smaller of the two "Large" sizes are about as big as you want to go, typically.
6. Paste the URL into the reply box on the forum. You do not need to add extra [img] and [/img] tags since Flickr already provides them.
So you get this:
PowerPlant 1 by
Chicagooan, on Flickr
If you don't want all the extra stuff around the photo ("PowerPlant 1 by Chicagooan, on Flickr"), you can strip out everything outside of the [img] and [/img] tags to give you something like this: