To make a screenshot, load your level, get Juni to the screen you want to capture, then press Alt-PrtScr while the KS window is active. That puts an image of the window in the clipboard. You can now load some graphics editor - even MS Paint will do. If you have Photoshop or something similarly advanced, then I assume you know how to create files from clipboard there. In MS Paint just press Ctrl+V, which will paste the image, then go File->Save and save it as a PNG file.
Then you create a topic in this board, KS Level Previews, and attach said image to the message by using Additional Options->Attach. As for uploading the level, you can use knyttlevels.com (choose "Upload New Level" at the left), or any filesharing service such as Mediafire, Sendspace, etc.