Simple (????) way around this if you're on windows:
- Hit he print screen button (which will tell the computer to copy whatever is on screen at that moment),
- Open up an image editing app like MSPaint or Photoshop
- Paste (ctrl+v) the image onto a blank canvas.
- Save it as a png or jpg file wherever you want on your pc.
- Come back here and under the reply box click the button that says More Options.
- On the following screen choose Upload a File, and select the picture.
Not familiar with other OSes, but I'm guessing the options steps will work pretty much the dame.