How do I post a screenshot from Mac OS X Lion on askdifferent?
To take a screenshot of the full screen hit cmdshift3.  
To take a screenshot of a part of the screen hit cmdshift4. You'll get a crosshair, click the mouse button to define the first corner, hold the button, move the cursor and then release it to define the transverse corner. The grey overlay spanning between the two corners will be the screenshot.
To take a screenshot of a special window hit cmdshift4 then the spacebar and choose the window with a symbolic camera.


The default location for saved screenshots taken this way is on the Desktop.

To change the default location enter:

defaults write com.apple.screencapture location /path/;killall SystemUIServer


Example:

defaults write com.apple.screencapture location ~/Pictures/


Hit the return key to set ~/Pictures as the location. You’ll need to follow it up with a SystemUIServer relaunch too:

killall SystemUIServer


If you want to save them in a new folder create a new folder first with

mkdir ~/Screenshots




To upload it hit the symbol pic (with the red circle in my screenshot) and choose your screenshot: