That's a good start :D
In Moonpig choose a card that allows you to upload a photo and click on the card.
Click the 'personalise' button beneath the card.
On the new page that appears, click 'upload your photo' in the centre of the card.
Click 'edit photo'.
Click 'upload from my computer'.
Choose the place where your photo - probably 'pictures' and click on the correct file.
That should be it