1.) Use the "Printscreen Button" on your Keyboard ( that will save an image of your screen on cache )
2.) Use a photo editor like "Photoshop CS2" to resize and crop your image ( limit it to 600 px width and save them in jpeg format )
3.) Go to Photobucket.com and make an account ( This is what we call an Image Host )
4.) Next Upload the Images on your newly created account
5.) Then you copy the IMG code on the thumbnail
6.) And lastly you paste that IMG code on your post =)
Happy Posting ^^