Flickr is easy. I will admit I had a good friend talk me through it the first time, long ago.
Open the Flickr page, click one photo (even the one you want) to open a slideshow. Right click on the photo, click "view page info". Scroll down, copy off the line (just right click and the whole line is highlighted) that has "staticflickr.com" in it, and paste it into your "insert photo" link on the forum page.
That works on every forum I have ever posted photos on EXCEPT this one, no matter what I do a popup box says "error: invalid file", which is is not, as I can paste that into a new window and it opens just fine.
https://c1.staticflickr.com/5/4263/35207120032_dd17b74f47_b.jpg
You try. That link opens a photo on Flickr of three postwar Lionel Geeps in NP "Canoe" paint, but will not insert into this forums software.
I tried removing the "s' from "https", I tried removing everything left of the c1, which is how it shows when opened in a new window, but not here.
Yes, it's secure protocol, but I tried without. It is an allowed .jpg.
Not complaining....I can still post the links readers can open, but they will not just "show" in the thread.