Most know how to do this but not everyone, there might be faster ways or better ways of doing it but this is the method that I've been using for a long time.

5 easy steps imo.
Using the Paint program & Imgur upload site (Account not needed) That's pretty much it.

1) Find the item you want to screenshot, hit the "PrtScn" (Print Screen) button on your keyboard.
(I believe if you're on a laptop you might have to push the "Fn" button as well)


2) After hitting "PrtScn", open Paint, hit CTRL+V to paste the image inside of Paint. ** Save photo afterward.
* you can click the "Select button" if you want to crop the item itself*



3) Go to Imgur site, click New post/upload (Top left corner) & upload your image "Chose photo/video"


4) Once uploaded, right-click photo, copy image address, take that link to your thread.



5) Click the image button (circled in red above), insert your photo's link & that's it
Example


Boom share your photo & sell a legendary key


Hope it helps at least one person out there. No more using your phone to take screenshots

You can use this same method if you want to upload to the photo gallery (you don't need the code, just the image address.