You won't be able to paste a screen shot directly into a RR forum thread. Your options are to either save it as a jpg/png/gif from Paint and then upload it as an attachment, or save it as a jpg/png/gif from Paint and upload it to a website like imgur, then put that link in the forum thread as an inline image.
To do an attachment, which is at this stage probably easier for you, save the image from within Paint, remembering where you saved it, and then in the "Advanced" reply function in the RR forums, go down past "Additional Options" to the "Manage Attachments" button; click it, and follow the instructions in the popup that comes up.