AMCWAS

Note that I used Mac OS X 10.5.8 (Leopard) and Windows 7 so I'm not sure how different they will be for older or newer versions.

WINDOWS:

Spoiler

To take a screenshot, press F2.

To find your minecraft folder:

Click on START (it should look like a Windows icon on the bottom left of your screen)

You should see a search bar that you can type into. Type %appdata% and you should see a folder called Roaming pop up.

Click on Roaming, and .minecraft should be the first folder there.

Click on .minecraft and inside you will find all your minecraft goodies. Screenshots is what you want, and if you took screenshots with F2, they should appear there.

Pictures so you can better see what I mean:

Spoiler

START

%appdata%

Roaming

.minecraft

 

MAC:

Spoiler

To take a screenshot, simply press fn+F2

To find your Minecraft folder:

Up at the top you will see a menu bar. The left most icon is the Mac apple. Make sure you click somewhere on your desktop background. To the right of the apple it should say Finder. To the right of that, it says File Edit View History etc...

Go to File > New Finder Window > Library > Application Support > minecraft

Double Click on Minecraft and you will find several folders. Saves is where your worlds are stored. Screenshots is where your well, screenshots are! Yay!

 

If you still don't quite get it, have some pictures (click spoiler to reveal):

Spoiler

FINDER

NEW FINDER WINDOW

LIBRARY

APPLICATION SUPPORT

MINECRAFT

SCREENSHOTS

Once you have your screenshots, upload them onto a site like photobucket, imgur or Flickr.

I like to use Photobucket for my pictures. In order to upload multiple files at once, hold down command and click all the ones you want. (it's ctrl for Windows) To post an image here, either right click on the image and select 'copy image location' or you can left click on 'Direct link' (located to the right on Photobucket, not sure of other places like imgur).

Now that you have the link for the image, you can either do what Wartower suggests (typing the code) or you can go to the advanced editor and you will see an icon with a tree on it. It's the 'insert/edit image' button, and when you click on it, you can then paste your link in there.

And there you have it. :)