You can upload the image from your computer using photobucket or imageshack. Once you upload from there you will get a URL. Just copy and paste that.
Or, if you wanted to use a picture from google or someplace, just right click the image, click "properties" copy the full address and paste in your post. Then hightlight the address and click the image icon (it looks like mountains and a sun) in your post under the color option. Or click the image icon first and paste the address in that box.