I use GIMP to scale my screenshots down to 680 x 383 pxl (for 16 to 9 resolutions) . I also tested Paint and the result is almost of the same quality, if you want to keep it simple
.After that I upload the picture to my personal artworks on Steam, copy the link after I opened it in a new Tab and use the link with the [img]-command here in the forum.
The preview will help you to check if it worked.
I hope this helps. I don't know if I can find the time for a detailed guide in the near future.