Yes once you have done the print screen, you then have to open MS Paint, click paste and then click save to save the file to a location on your hard drive. By default Paint will try to save it as a .PNG file, but for this one, of what I get when doing a share from Flickr I chose to change it to a JPEG.
So this is what I get when I click the share icon in Flickr
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I then use Ctrl+C to copy the BB Code, since it is already automatically highlighted you don't need to do anything else first. Then back in the POTN editing box I just use the Ctrl+V Key combination to paste the link. This is what it will look like in the Editor.
[URL=https://flic.kr/p/FBy56T][IMG]https://farm2.staticflickr.com/1684/26000248693_89e4a97d28_o.jpg[/IMG][/URL][URL=https://flic.kr/p/FBy56T]Gloster Gladiator Mk I, Hawker Demon[/URL] by [URL=https://www.flickr.com/photos/alan-evans/]Alan Evans[/URL], on Flickr
Note that in the edit box it will probably be broken across three lines. And this is the image that the link will display: [IMAGE'S LINK: https://flic.kr/p/FBy56T]Gloster Gladiator Mk I, Hawker Demon
by Alan Evans
, on Flickr
I'm using Chrome as my browser, but it will work the same with any browser you like.
There is also a new tool in Windows, called the Snipping tool, that will allow you to copy bits of what you see on screen. I'm old school, and so still stick with the Prt Scr and Paint option, as I haven't learnt the new tools functions. It's not something that I find that much need for.