I know this has already been done before by Noobkilla and Zambiana, but I use a different method.

WINDOWS:
This works for Windows 8 also.

1. Make a new folder on your desktop and name it whatever you want. (I named it "Minecraft Screenshots")

2. Click "Start" and type in %appdata% OR hold down your "windows key" (On the right to the CTRL key) and "R" key and type in %appdata%.

3. Go to .minecraft/screenshots

4. Move all the pictures into your folder you made in step 1.

5. Go to the forums and click "Upload a File"

6. Find "Desktop" then open your folder with the screenshots in it.

7. Choose your file and it will automatically be uploaded