Look under your System Preferences -> Keyboard & Mouse -> Keyboard Shortcuts you should have a Screen Shots group where it shows you what shortcut has been assigned to different screen shot types.
The one you want here is : "Save picture of selected area as a file" (Sorry I changed my default seetings on this so I no longer can give you what it should be). Upon invoking this shortcut, you will be presented with a "crosshair" marker, simply selected the portion on your screen you want to show us.
Once your image file has been generated, you will need to make it available to the web somehow (either through a personal website or a web picture sharing utility) so that you can reference to it in this board (You cannot Copy/Paste an image in here).
HTH