For flickr you have to do a few extra things, and honestly I wouldnt even advise using flickr for sharing images, it's much easier to use imgur... however if you do want to use flickr, do the following
#1 Click the "share" arrow, this will bring up the next screen
#2 copy the link they give you, under "BBCode"
Paste that into the forums and you will end up with this;
Car Polish and wheel powder coat by
paul crittenden, on Flickr
Note that the code for that is this;
Code:
[url=https://flic.kr/p/ZYbhfP]
[img]https://farm5.staticflickr.com/4528/38048065355_3f33a674a5_z.jpg[/img]
Car Polish and wheel powder coat[/url] by
[url=https://www.flickr.com/photos/156074127@N08/]paul crittenden[/url], on Flickr
If you just want the image copying and none of Flickr's fluff, you just want the stuff inside the [img ] tags
Code:
[img]https://farm5.staticflickr.com/4528/38048065355_3f33a674a5_z.jpg[/img]
Note that the link is pointing to a .jpg, which is an image format, if the link doesnt end with the image format, .png, .jpeg, .jpg etc, you havent got the correct link, it needs to be the direct link to the direct image file.
Hope this helps