It's an OS function.
Try this out now:
Hold down shift-Cmnd and press 3.
You should hear a camera like click sound and on your desktop you'll see a new icon "picture xx".
You can do this within any application too.
Try out the other key commands (shift-cmnd-4) as this will allow you to capture sections of your current screen.
To upload it to your post you need to place the photo on your website or any picture sharing site (i'm still looking in to this).
Copy the internet address of where your picture is stored.
When you write a post, at the bottom of the text input box there are a few options (you can italisise or make text bold, add an image or a link).
Click "add image" and type in or paste the address of your picture.