I'll assume you have already taken the screenshot, yes?
Well, open the .bmp (bitmap) file in a paint program of your choice, MS Paint that comes with MS Windows if necessary, and then resave it as a .jpg file.
From there, you can either upload it to any webspace that you have and use the [ img] links explained in the
vB code link above, or you can use the "attach file" feature that's included at the bottom when you compose a post.
Simple, eh?