Both buttons do the same thing. However I don't use these, as they by default give you thumbnails linked to images with ads.
There are three ways to get a full-sized image directly into your post using Postimage.
I've covered them here.I'd recommend the second or third method. Both involve creating your own account at Postimage first.
Method 2: just login to the Postimage site and click the 'upload' button, Upload your image, copy the 'direct link' to the clipboard (there's a handy copy button), paste the link into your NZFF post, and add the image tags.
Method 3: install the Postimage Windows App, right click on your image in Windows Explorer, Send to | Postimage, it'll open a window showing the upload process, then you can choose to copy the direct link to the clipboard, paste into your post as usual.
Method 1 gives a thumbnail rather than a direct link, so you need to do some fiddly work to get the job done. Easier to control it yourself.
I do like and recommend the Windows App, it has some powerful screenshot features I use often, mainly for tutorials, like selecting part of the screen to upload/copy/edit, adding annotations, mark-up etc. You can set it up to 'hijack' the prtscn button, on my system it greys the fullscreen and I can draw a box around the area I want, then I get to choose whether to edit, copy, upload, save or print the selection. (Cntl-A Select All works as expected to get the whole screen.)
However for flightsim screenshots, using 'v', I resize them in PhotoShop, then save and upload using the right-click method in Windows Explorer.