I've seen many program pages on SmileBASIC Source that use images of the screen taken with a camera.
You can get much better images if you use Miiverse to take screenshots! (They're still JPEGs, sadly)
Get the image you want displayed on the screen.
Press the home menu button and open Miiverse.
Select the screen you want, and choose "Save to Screenshot Album".
Log in to Miiverse on your computer, and go to the bottom of your user page.
Click the screenshot you want, then right-click and choose "Copy Image Location".
Set the image on your program page to that URL.
(Note: I'm pretty sure this doesn't work if you're banned from miiverse, so you'll have to ask someone else to do it for you.)