the best way imo is like this: If you found the picture with google, right klick on it ans selcet properties. A window will open. There you have to copy the Adress (URL) and paste the url into your post .Add [img ] at the beginning of the url and [/img ] to the end of the url without the space in it.
thats it.
p.s: I thought there was a tutorial how to add screens into your post, but i couldn't find it.